rumbling rodeo

I've got a 92 rodeo 6 cyl 4wd with about 180000 on the clock. ive replaced the clutch package, the u joints and the rear wheel bearings. when starting off from a stop i get a rumbling/vibration from the drive shaft or rear end, the bellhousing bolts are tight, as is the center bearing.motor mounts and tranny mount seem good too. vibration peaks at about 45 mph..... strangle.... when reversing there is no vibration at all?? any ideas professors? its got me stumped, dont wanna even think about the diff!! bill