Keep your truck.
One step I’ve always taken with a used (new to me) car is to change every fluid. Engine oil. Trans fluid. Differential oil. Transfer case oil. Coolant on your truck is 100,000 mile/ten year. No worries on that.
So, change them all and establish a new baseline.
The Mobil 1 EP 0W20 is a good choice. I’ve been running Pennzoil Ultra Platinum in mine since it was out of Toyota Care. Do a couple of shorter intervals, like 5,000 miles. Then choose a good oil and go back to 10,000. If you short trip, or any other hard use, stick with 5,000.
If you are interested, HPL, which costs more, cleans well, and can be run for longer. If you go that route (and I will with my Tundra when I use up the stash of PUP I got cheap) then change the filter once at 5,000 and stick with a 10,000 mile interval for the oil. 10,000 for both after that. No sense throwing out perfectly good, expensive oil.
Get a tool for the filter housing.
Transmission takes WS. You’ll need 14-16 quarts to do a complete fluid change via the cooler return line. Tundra forums have details on this. Not hard, but you’ll need Techstream (or an IR gun) to get the level correct when you’re done.
Differential takes 75W90. Lots of good choices. I have AMSOIL in mine now, which will get changed over to HPL when I get to 60,000 on the AMSOIL.
Transfer case is an odd duck. It takes a 75W. It’s synthetic manual transmission oil. You need 1.6 quarts. Toyota will sell it to you for $80+ a quart. That’s not a typo.
Two other options: Ravenol for just over $20/quart. HPL for a bit under $20. Again, I am an HPL fan. See below.