Reliability varies with the 4R and 5R transmissions depending on a lot of factors, but I have had nothing but good experiences with them personally. I am not really easy on my trucks either.
An '08 should be as good as they get...updated valve body, and still has a dipstick (I think).
My 2002 Ranger 3.0L has 185K miles on the original, unrebuilt 5R44E. I have made absolutely no repairs to the transmission. It has had one computer reflash done around 2012 that updated some things for the transmission. It has had one pan drop and filter change in its life, but mostly regular fluid changes via cooler lines or dipstick tube. The pan was spotless when dropped for the first time around 148K.
My truck's transmission has probably done somewhere around 15-20K miles of towing trailers in the 3K lb. or so range. I'd drive it anywhere, but I do think the trans is beginning to show it's age just a little. It does seem like once in a while a shift is slightly slower to complete. Not really a flare or slip, just some extra sloppiness it didn't always have. Not enough to keep me from hammering on it, but I know one day the time will come where it's an actual drivability problem and the trans will need to be rebuilt. I'm thinking that will be after 200K, but exactly when is hard to say. I'm not really concerned about it.
Between 1995-2001 there were issues with valve body gaskets blowing out and causing shift flares and other drivability issues. It's not too bad to fix, but I think many shops diagnose it as "needs a trans rebuild" for fear of being wrong, or just not being familiar with these transmissions. 2008 is way past that though.
My 1994 Ranger 4X4 has an M5OD manual. It functions, but I'd actually rather have an A4LD auto in good shape sometimes. The A4LD is often considered the worst automatic Ford put in the Ranger, but it could be built into a decent trans and I have seen many with higher miles that work fine.
Every Ranger automatic I have personally seen with significant problems at less than 200K miles or so had poor maintenance history and nasty fluid. Every single time. Not saying there aren't bad ones, but I do think maintenance plays a big role with this model of transmission. 55K miles isn't much, so you should be fine. Just change it if you don't have any receipts for it being done before.
