I have an 88 Thunderbird 3.8L. When accelerating off the line it feels as if I am running over a small pipe on the road. That's the only way I can explain it. It's easy to duplicate and only happens when you stomp on it, at least half throttle off the line. I don't see anything wrong visually. I took the drive shaft loose the other day and the U-joints felt OK, one was a little stiffer than the other on the front end, but none were bad. I can only visually inspect the motor mounts, but they look OK and the engine does not rock in the engine bay when revved. I noticed the other day that a small amount of smoke from between the firewall and the engine will be visible on rare occasions. The engine does not leak or have any other problems. When driving normally everything seems to work well, no sounds, bumps or other signs of misbehaving.