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Recently did an Engine swap in my 1995 Grand Prix SE, it's seen a very rough life, body has 153,xxx miles, the most recent transmission has roughly 75,xxx miles, and current engine has 51,xxx miles, all of which I've kept close tabs on. My problem comes from the recent engine to transmission matchup, in which it shifts, accelerates, and overall drives like factory new, except for overdrive. When I hit overdrive, there is a shake that's not quite alarmingly bad, but is definitely noticeable, and absolutely ONLY when overdrive is reached. I was in a bit of an aggravated hurry pulling the previous engine with a cracked head, and in my hurry, I accidentally pulled the still-attached torque converter with it. I unbolted the Torque converter from the flywheel and slid it back into the transmission, but I almost always mark the position in the transmission to ensure proper realignment, which I didn't have the chance to here. My question is would an imbalanced torque converter potentially cause this shake? I really do hope that is all I have to fix.    

If it was a balance issue it would be sensitive to engine rpm's not overdrive. The most likely scenario is that the tcc clutch in the convertor is chattering. If you hook the car up to a good scanner that can display tcc duty cycle data you can  see if the vibration coincides with tcc apply. You could also try getting some limited slip oil from any GM dealer. It comes in a 5oz squirt bottle . Squirt about 1oz into the dipstick and see if the vibration  gets better or goes away completely.

Did i read this wrong? Is this the same trans that was with the other motor?  So this trans didn't have this problem before you changed the motor? Then maybe its not the trans. Have you looked at your spark plugs? If the gap gets to big you can get an engine vibration that only shows up in high gear or lockup.  

Yes, you read that right, the transmission was in the car and ran flawless prior to this engine. I never even thought to look into the spark plugs, but now that you mention it, I can remember a few instances where that was the case. Thanks very much for your helpful reply!