I just noticed this last night on my way home from work..... My '91 has, what I can call, a "thunk" when ever I accelerate from a dead stop. The strange thing about it is that it doesn't happen right away, it happens maybe .75 second (3/4 of a second) after I hit the gas from a dead or near stop. It doesn't matter if I apply the gas lightly or HARD! I've already checked the obvious; ball joints, tie rod ends, etc. It does happen in 2-wheel drive, but happens more often or more strongly (loudly?) in 4-wheel drive. Could this be a bad U-joint in the front axle? my rig has 140,000 miles on it!