Ill start at the begining, I have an 88 GTA now and since have been doing some work to it to fix it up. I noticed that the trans mount was torn and was causing some harsh thuds under the car so I decided to replace it with an Energy suspension piece. I got the car all back together and on the test drive to make sure it was ok I noticed its now vibrating when the engine hits about 2k rpm and then again at around 4k and up to redline. I was wondering what could be causing this vibration in the engine and/or what I can do to make it go away. I knmow its engine related because it even does it sitting still when I free rev the engine to these RPM ranges.

 

I had a similar problem and in the end it turned out that the new transmission bushing slightly changed my pinion angle.

The vibration would start around 60 mph, ~2000 and just get harsher the faster you went. At 80mph and above on the freeway it was just awful.

 

I dont think its driveline related since when its doing it on the road I pop it in neutral it goes away when the RPMs drop and I can actually get it to vibrate when its sitting still and rev it up. I was thinking along the lines of harmonic balancer and since this mount is much more solid its not absorbing the vibration but I would like ot know where else to look and what all I should check for as well.