2002 3.8l 5spd. Just purchased vehicle a week ago here my issues. As I change gears I get a vibration as I release the clutch and it transfers power. The other issue is vibration while driving. Worse around 40-60mph in 4th or 5th gear. If I let off gas it eases up and goes away same if I push in clutch. Which rules out wheels and tires. I was also told that rules out driveline issues like u joint and drive shaft. My first guess was universal joint at transmission but since it stops when I let off gas or push clutch in I assume it's something diff. When setting at an idle and revving engine it vibrates some but I'm not sure if it's worse than just normal engine vibrations. Could it possibly be harmonic balancer?

 

With the car running, open the hood and have a look inside. Try revving the engine a few times as you watch the motor. If the motor is abnormally shaking it could be a bad motor or transmission mount.

 

Another method that may reveal more than just reaving is to put it gear(easier with an automatic) hold the brakes then goose it a little. If the engine moves more than just a touch, you have a bad mount... Do the same in reverse to check the other mount. Side it tips too is the bad one.

Though in your case I doubt that is the problem. I would suspect something more to the effect of the clutch just needing to be replaced. Possibly U-joints, but I doubt it, though they are easy enough to check and probably should be checked before you take the transmission out.

 

Engine/Tranny mounts and/or U-joints. I'm going to go with Mounts first. Any bad clunking when you let your foot off the clutch when shifting? That would be more indicative of a u-joint.