[davepl]

Gotta be between the trans output shaft and the tires, so perhaps U-joints, carrier, or less likely, the differential. But the good news is that diffs usually just whine... if it were grinding I think it'd fail pretty quickly and catastrophically!

My best guess though is the carrier bearing, they're a common failure point.

If your truck has a lot of miles on it, sure wouldn't hurt to throw all new U-joints and a carrier bearing at it. They're service items anyway, and then you'll know they're good.

If you get u-joints with grease fittings (zircs) be sure to put them in compression, but tension (so that when the truck engine tries to turn the driveshaft, it's squeezing one of them, not pulling it apart). Not a big deal but a minor thing that can help longevity under higher power applications, anyway...

PS: Nice looking truck!