Maintenence records! Ask to see bills for repairs as well as basic stuff like oil changes and transmission fluid changes.
Also, wipe off the dipstick and check for varnish. A lot of varnish could indicate that oil changes were not done as frequently as they should have been. Check the other fluids as well for any extreme discoloration, foaming, crud, cross contamination, etc... You should also look under the truck for major leaks (there will be small leaks on an old truck), rot on the body and frame, and the general condition of the important parts of the truck.
Drive the truck a good bit as well on different types of roads (highway, in town, slow speeds, faster speeds). Let it get warmed up and look for a check engine light.
Damage/excessive wear is not always a reason to pass up a vehicle, but it is good to know just what you are getting into and make sure that any problems with the truck are ones that you could either fix or learn to live with.
You should also ask if the truck has ever been overheated. 2.9Ls do not like to be overheated, and it sometimes results in warped heads.