Put the truck on jackstands under the rear axle so the suspension and driveline are in driving relation to each other. Then look underneath when it is in 1st gear. My guess is it may be a drive-shaft phasing issue if you took the 2-piece driveshaft apart and did not put it together in the correct phase.
you could also have a bum U-joint, output shaft bushings from the trans, or pinion bearings from the rear diff.
Good Luck.
