They guys have given you several things to look at. Now it is time for you to crawl under there and take a good look. Recheck those u-joints. Pull off the drive shaft and check for play (radial and in/out) in the pinion. It may be time to drain the tranny or rear end to see if there are chunks.

Remember, you are working with a 20-some year old truck. Things loosen up over time and while nothing maybe "loose" the additive effect of all the wiggles will make for some strange behavior.

Just to check, this is something under normal driving conditions, right? You aren't accelerating, letting off and then stabbing the gas, right? I can get any vehicle to lurch or jar by building up slack in the cost side of the entire drive train and then flooring it.

Good Luck,
Michael