Now I have this same vibration in the gas pedal, it comes on after driving a bit, if I place the transmission in neutral the vibration ends immediately. This is a new phenomenon, came about after having the transmission worked on a few weeks ago. Reverse had quit working, I took her in to have a "reverse clutch" repair, it worked, saving me much money, but resulted in this new vibration. I also had the rear hydralic automatic levelers rebuilt, and new engine mounts installed. She's a very nice 91 560SEL 126. I also now hear a faint clicking sound when backing up.
Its nice to find this forum out there in the abiss.