vibration problem

Sooo, I ended up putting 3 degree shims in the jeep as well as new U-bolts and motor mounts. she is completely vibe free!!!

Can't say the same for the duster though. I balanced the wheels, checked out the undercarriage (nothing loose), the bearing on the input yolk for the diff feels excellent, wheel bearings look good, jacked the rear end up and put her in drive (no run out in the wheels, straight axle), new u-joints and balanced and straightened the drive shaft, d-shaft angle at the diff looks perfect, out-put bearing on the tail shaft of the tranny feels tight and good, motor and tranny mounts are new....

Vibration hits about 50mph, i can feel it in my seat and the floorboards, not the steering wheel, doesn't go away when i throw it in neutral at speed, doesn't vibe in neutral or park while revving. after i hit that speed it only gets worse the faster i go. if i let off the throttle it doesn't get better, until the car slows below that speed again.

It has a very low pitched, constant oscillation to it (humm, HUMMM, humm, HUMM...) gets quiet and then louder, more violent then it calms. at about 50mph im taching around 3200rpm, gears are 488's..... i think i can faintly hear the same osculation sound at lower speeds but not sure.

Any thoughts??? I'm kinda outta ideas of places to look. My next thought was that the gears were toppin out in the rear end or something was on its way out back there???
Any info or progress on the original or other vibes in this thread???