In 4th gear, when it gets above 50( I think when th converter locks up)and I floor it, the car vibrates badly. It only happens in 4th gear. Any ideas what it could be? I was told maybe a cylinder dropping?

 

well...i was gonna aim at the driveshaft until you said was isolated to 4th gear. Lt1 driveshafts done seem to be balanced nearly as well at the LS1's. maybe you've got a wheel out of balance. Let me know what you find out, im kinda curious.

 

Does it give you a quick jolt of vibration, then runs normal again? or does it continue to vibrate? if its the first one the converter is locked up, and is unlocking, mine does the same thing.

 

Mine does the same thing at about 68-72 mph. I have a third gen with a LT1/4l60E swap and the LT1 driveshaft. I think mine is an alignment issue at this point though (or at least I hope).