1986 420sel advice needed.
 

My car is a 1986 420sel. I do not know the exact mileage on the car, but I am to guess its about 240k. This car is my daily driver, and by monday, will be my only car. I have had driveline slack issues since I bought the car back in 2006. The only issue I used to have was that it slammed into reverse. My engine started idling at about 1k for a while and I finally got around to replacing the ICV in December of last year. That made my car go into reverse pretty smoothly most of the time but not like I think it should. As I have owned the car, I have noticed that if it turn a corner and then give it gas, it can hear it clunk as if the driveshaft is loose. This also happens when say I am on the access road and I slow down for a car turning right in front of me, and then I give it gas after slowing down I hear the same clunk. Last year I decided to replace the flex discs,center bearing, CB support, axles, differential mount and transmission mount. Two weeks ago I replaced the engine mounts. None of this changed much except now that it is safe to drive the car as the flex discs were very bad. I put the car in the driveway tonight with the parking brake on and in drive. I could not move the driveshaft by hand side to side or up and down. I put it in neutral, and it could move side to side. Put it in park, and it turned a little just once and then it would not move at all. I am not sure if that is normal. Also, when slowing down, almost coming to a stop, I can hear and feel the driveshaft making noise when the car downshifts each gear. How can I decipher if I have a transmission issue or a rear end issue? This is driving me nuts.

i would check out the differential mounts.sounds like the left side one may be failing.