This morning I took my 1988 Suburban half ton, 4wd with 350 out for a few errands. Went about 5 miles about 50 mph. Had to make a u-turn didnt quite make turn so had to stop go into reverse. At this point I heard a loud thud and jerk in driveline. Went forward and heard and felt rapid thuds seem to come from front. Headed for home thuds went away and cruise home at about 55 mph. Stopped in front of house and stared back up and thuds again. Seems from front end, front end was visibley bouncing. Fairly rapid. Almost sounds like a u-joint. Looked at front and rear driveshafts u-joints look good. Noticed that I can turn the front drive shaft by hand nearly a quarter turn with vehicle off and in park. Is quarter turn ok ??? Other info, replaced rear transfer case seal and driveshaft about 20,000 miles ago - no problems since. Also a few days ago took burb on 400 mile round trip. Got bad gas at beginning of trip and about 75 miles into trip started having fuel problems. This was in remote area and speed was reduced to about 35-40 miles an hour for about 75 or 80 miles. Got fuel filter and problem solved. Drove rest of trip with no problems. Could something in driveline such as transmission, transfer case have been damaged ??? I dont see how, but maybe ??? Any ideas on how to trouble shoot this etc will be appreciated.