coincidentally i just replaced the rear flex on my 90 300te 4matic.i also dropped the driveshaft to check the center bearing support.the easiest way to get at all of this was to drop the exhaust system.my floor jack gave me a 20" lift.at this height i was able to work under the car comfortably and the exhaust dropped right down to the point i could drag it out from the rear.the two pipe sections between the cat and first muffler were quite rusty but a solid tap with a hammer indicated a defered repair.inspection of the driveshaft and center bearing indicated no problem.so i started looking elsewhere for the cause of my driveline vibration.i noticed that both front and rear flex discs were similar contrary to info in the service manual so i replaced the rear with an older version that i had on hand.also noticed quite a bit of movement at the tranny mount.not having that part on hand i guess well have to wait.