Any idea what will cause really heavy vibrations around 1800 rpm under load? If I downshift or let off the throttle, it will go away. Only occurs under more-than-average load, like going uphill or accelerating.

I'm thinking transmission mount? Haven't had the time to get the car up in the air and look at it yet, just wanted to ask if there is a well-known issue that will cause this..

 

Check your U-Joint and make sure its properly seated in the bearing cup holder.