A point of information regarding "locking" rear axles. There are basically TWO types of these rear axles--one is locked until it unlocks and the other is unlocked until it locks-up.
In the "locked until it unlocks", there are springs (sometimes four small coil springs or one S-shaped spring) that keeps the axles locked together until there is a need for slippage to take place in the clutch pack (i.e., going around a corner). This is how all GM car and light truck G80 "PosiTraction" rear axles were up until the middle 1980s-era. These rear axles NEEDED the GM Limited Slip Additive (after the particular-part number GM rear axle lube was replaced by normal lube + additive; by that time, all normal rear axle lubes were spec'd for limited slip rear axles, by observation). It was not uncommon for the rear axle to "pop" on a left hand turn in cities and such, even with a "binding and then release" feel to the vehicle--which pointed out the need for a fluid change with some new additive. Typically, cars and 1/2 ton trucks took one bottle, but 3/4 tons might have been better off with 2 bottles. These were all option code "G80" limited slip rear axles.
When GM started talking about "G80 Locking Differential" in their option lists, that indicates the "unlocked until it locks" version of rear axle. This might sound funny, but the axle operates as an open rear axle until a wheel speed differential between the rear wheels spins a governor gear internal to the rear axle carrier's internal gearset to orchestrate "locking" of the two axle shafts together. There are clutches, typically, but the later Eaton G80 used (generally exclusively on GM light duty trucks, including the S-chassis) mechanically locks things together.
In the CD video demonstrations that GM has on the "G80 Locking Differential" (sometimes mentioning Eaton's name), there are two variations. One is a 1/2 pickup trying to tow a boat up an incline--one wheel on wet, soapy surface and the other on dry pavement. Only the GM truck makes it to the top as others spin and have all sorts of trouble. The other one is where the trucks drive across a special culvert to get one of the rear wheels out of contact with the pavement, or barely touching, as the trucks stop and try to restart. The other brands will spin only one wheel and smoke the tires, but when the GM truck drive up and stops in the same place, with the G80 Locking Differential (Eaton), the free-er wheel spins slowly and then the whole truck shakes as the locking action takes place, then the GM truck drives away normally.
Seems like it was the middle 1980s when the Limited Slip Differential Additive was "not recommended" for GM light truck rear axles with G80. This was a GM TSB, which stated that it could degrade clutch material life if used in those vehicles. Yet other GM vehicles with normal G80 axles were supposed to use it--read the owner's manuals, I guess, or don't use it unless there are "binding" problems on corners.
In the later 1960s, there was also a mechanically-locking rear axle known as "Detroit Locker" or "PowerLock". It did a mechanical lock rather than lock-up the cone or flat clutch plates in the differential carrier. A mechanical lock when two metal, toothed cogs firmly engaged. Other than some of the more powerful musclecars of that era, they were also used on GM's heavier-duty light trucks.
There are some other aftermarket verions of limited slip or locking differentials out there too, which can have some different fluid requirements by their manufacturers (some which might not agree with those of the vehicle manufacturer when the "tuner" vendor might put them in (i.e., SLP SS Z-28s and similar Firebirds with the Auburn rear axle unit).
Now, for lubes and equipment changes . . .
When GM went to their synthetic rear axle lube recommendations, their light truck 9.5" ring gear axle had never used a gasket for the rear cover, only silicone sealer--ever. The year that synthetic fluid was first spec'd for the rear axles, they used a different rear cover and A GASKET. A TSB stated to not use synthetic fluid in an earlier 9.5" "gasketless cover" rear axle as the synthetic fluid would degrade the silicone sealer over time. It also stated that to use a gasket on a prior year axle, you needed the appropriate rear cover to go with the gasket. From what I understand, there should be a tag on the rear axle (during model year transition) for the type of recommended rear axle lube (as the 1/2 tons apparently used the synthetic lube first, then the heavier-duty trucks came later), other than the traditional PosiTrac Lube plastic tag. IF the rear axle has the noted drain plug on the bottom (as the later versions do), all the much easier to do things.
Other than the rear cover gasket issue, I'm not aware of any other internal changes made when synthetic lube was first spec'd on those trucks, but it could be that other seals and such were already synthetic lube-ready. Possibly some clearances could be different, but I somewhat doubt it.
It could well be that fuel economy was a consideration in going to the synthetic lubes, but with the same vicsocity ratings, that might be questionable. Lubricity CAN be an issue though, plus longevity.
Some dealers still recommend changing the rear axle fluid under severe use conditions as they did before. This might sound a little flaky, but modern light truck users (especially the diesel owners and 1-ton owners) are using their trucks to do basically what medium duty trucks used to do. Therefore, where "no change" might have worked for ages (other than with the PosiTrac issues noted), more prevalent heavy loads for extended periods of time can degrade the additive packages in the synthetic lubes too, I suspect. Cheap insurance to prevent castrophic axle failures that result in downtime and loss of revenue or other delays--whether synthetic or normal lubes.
The Eaton G80 Locking Differential (whether 1/2 ton or heavier) can be easily identified when the rear cover is removed. You'll see a thin, vertical toothed-on-the-circumference gear that will work a teardrop-shaped governor weight on a related shaft. This governor set-up allows the axle to be "open" until there is a need for it to be "locked".
When the rear cover is removed, the G80 Positraction can be noted by the presence of the spring(s) inside the rear differential carrier, which keep the clutch pack engaged until it need to slip for going around corners. These G80s WILL need the additive to let them slip when they need to. Somewhere, it was noted that only about 80% locking takes place in this sort of "limited slip" rear axle.
Also, the locking action in the limited slip/locking rear axles results in the oppposite torque reaction of the rear axle housing. This means that rather than the passenger side rear tire "lifting from engine torque" upon intense acceleration, the driver's side rear tire will be the more lightly loaded of the two (with, generally, more black marks on that side than being equal, in the same acceleration mode). When in a corner, intense power application will make it easier to spin the left rear wheel in a left turn whereas in a right turn, the added weight shift in a right hand turn will keep everything more firmly planted to the pavement. With the "locking differential", the torque reaction would possibly still be there, but the "black marks" should be more even, I suspect.
Just some thoughts which might help clear a few things up,
CBODY67
