Did you get a chance to check the rear wheel (axle) bearings? Here are thethings I'd check if it were my truck ...maybe you checked some of these things already, but:
1) Rear axle bearings: (on jackstands, wheels removed), lift up/down on axle flange. Shouldn't be any or much noticeable play.
2) Driveshaft: Trans in neutral, lift up/down vigorously on rear of driveshaft. Pay attention to the u-joint and pinion for any movement. If that feels ok, wiggle the front of the rear driveshaft by the transfer case (should be tight). Wiggle vigorously any center support bearings/bushing (if your truck has a 2-pc driveshaft). The center bearing is a known wear point that can rumble.
If this all feels tight, I would recommend removing the rear of the driveshat U-joint from the pinion, and rotate the u-joint caps by your fingers, and rotate the pinion (by hand), feeling for any roughness. Make sure when you rotate the rear pinion the driveshaft is removed and tires are off.
3) F. Wheel bearings: Jack up front wheel (leave wheel on tk), wiggle wheel vigorously with hands on the 6:00-12:00 position. Should be NO play. Push/pull in and out on the top of the tire and there should be no play (checking upper ball joints). Rotate the wheels and listen for any 'funny' bearing sounds. If it's early in a front bearing failure you may not yet hear or feel much.
4) Front Driveshaft: With axle pre-load released, wiggle front driveshaft vigorously, feeling for freeplay or roughness in the joints or splined slip joint.
5) Shocks: Inspect shocks for any oil leaks ...in case what you're hearing is wheel-hop.
6) Tires: Run hands carefully around the tires feeling for a bubble, broken chord or unevenness. If you think it might be tires, having them Road Force balanced would be a great way to catch tire-related problems. Road Force balancing is on a special Hunter balancer (the best).
Good luck!!
