Which motor mount would go first RT or LT
 

I have a 91 300 E that I am hunting down a vibration problem while in D w/ the break on. I just finished replacing the trans mount and it's still there. It doesn't do it in Reverse, so I am assuming the engine is twisting on one bad mount and bottoming out against the bolt.

Which mount takes the most abuse? I am going to replace them both, but I want to do the worst first so that I know I am on the right track.

The one with the most oil on it will be the one you should replace first. On my 300D it was the one under the turbo that was pretty much gone :)
Look underneath there and see which one has the most oil/dirt on it. The oil causes them to deteriorate.

On a 300E . the right one usually bakes first from the exhaust
manifold heat.
The right has the torque on it , but the flexing is actually good for it.

You can measure the clearance [ [13 mm =1/2 "] between the cross member and the flange on the mount. Less than that, and they are collapsed and should be renewed.