this is something that just recently started happening soon after i changed the fluids in my diff. Not like right away, but its enough to annoy me now. so here are the symptoms as best as i can describe them:

-the car will shake under acceleration when in gear
-the car stops shaking under deceleration when in gear
-the car shakes when coasting and not in gear/clutch in
-the car doesn't shake when engine free revs

so i have basically come to the conclusion that its not the engine it self, it has to be somewhere in the drivetrain more than likely, but i don't know where to start so i thought id see if any of you have had similar problems and had an answer.

oh and the clutch was replaced a few months ago, it was just a clutchmasters stage 1, nothing special

 

could be your drive shaft. i had sort of the same problem except for "-the car shakes when coasting and not in gear/clutch in" that never happened in my situation. but once i swapped out my drive shaft it ran smooth.

 

ive had a situation where i had a vibration problem which i thought was coming from the rear end. turns out the bracket to the air pump broke making the belt loose as hell causeing the belt to vibrate when accelerating. . . . . dont no if it helps you in your situation

 

check your Front differential mount..maybe check the engine mounts and Transmission mounts while you have the car up on stands also.

 

ill check on the mount, thats makes sense to me. last time i checked everything was tight in the engine bay but i will go over that again in a bit. its probably the mount since when i let off it puts force on the diff and that could be why its not vibrating when its in gear and i let off the gas

 

i think i may have found the culprit behind the vibration. i jacked up the car a little while ago to check all the tranny and diff bolts and everything was still tight from last time i changed the clutch so i started to think that maybe it was something in between the two. well i let the ebrake down and put the car in neutural and went down to check the ujoint and sure enough it had a little too much play in it. then i started thinking that maybe it was from last time i went to the road course, i might have been a bit rough on it trying to clutch kick around some of the turns. oh well
so tomrow i will take the old shaft off and swap it with one that my friend has on his na shell since he is getting ready to do a tII swap and won't need it.

 

even though you were wrong at first, my air pump JUST broke off lol. the belt snapped and i was going to loosen the bottom bracket so i could swap another belt on and CRACK...the top bracket broke.

 

Check your drive shaft, my GTUs is doing the same ****, just get a new one,

also you can test it, lift up the car, and go under neath were the drive shaft is at, and see if it wiggles side to side,


your u joints are probably going out.

 

After reading this post I remembered the vibration that developed around 45 mph under acceleration.
Did my homework in this form and all pointed to my 20 year old stock driveshaft.
Added it to the list of things to do on the 7.
The car still has bumps and clunks in the rear (bushing kit on the list)
though the driveshaft vibration went away in time, forgot about it really.

Not sure why, didnt change anything under the car.

 

I had the same symptoms as you.

Found out it was my drive shaft.

Switched it out and ran smooth again.