Gotta '98 3.0L 4x4. Since I bought it last year I've noticed a high pitched whining sound ONLY while accelerating, which seems to come from beneath, possibly at the transfer case. At first I thought wheel bearing, but the second I let my foot off the pedal it stops. I've put about 2000 miles on it since last July, and the sound has steadily gotten louder since then. I'm concerned it's presaging a serious issue -- anybody got any ideas or am I just jumping at shadows here?

 

Could also be the rear end. If previous work was done to the rearend (or a failing pinion bearing) could cause a whining noise under load (acceleration).

First thing i would check is the oil levels in the rear end and the transfer case. Second, inspect U Joints for play. Third, you could take it to a shop with a lift and have it looked at with vehicle in air and in gear while someone is in the truck and someone else is below listening for exact source of noise. Im not suggesting you do this at home due to liability issues.

 

Copy that. Rear end and pinion have crossed my mind before as well, but I have zero experience with that stuff. The backplate of the differential looks like it's been swapped, so someone's done something to it.