I posted about 2 months ago a vexing problem I had been having with my daughters 245.
http://www.brickboard.com/RWD/index.htm?id=1138671

I replaced the RR TAB and all the Torque arm bushings, to no avail.
Although I could discern no play in them, I thought I might have to replace the U joints.
Winter is finally beginning here and a few weeks ago I put on the 195/70-14 studded snows. I have four but initially just put on the rear ones as I had taken off the summer wheels to fix the bushings and brake pads.
At first I didn't realize the change. Then my wife and I borrowed the car to travel in some predicted bad weather so i put on the front snows also. I was expecting some icy conditions and the studs would be the ticket therre.
Then suddenly I realized as we started out, NO R&V ! It was gone. The tire change completely eliminated the problem.
SO, the rear tires were 205/70 realy off brand. Their wet weather performance was poor, even up against the 185/70 GY Aquatreads on the front. I don't like running differing sizes, but was just wearing out these old tires. Even though the Aquatreads were narrower, they were still better in the wet.
Are the rears just faling apart internally and causint the vibration, or is the difference in diameter fromt he front to the rear tires?
Well, problem gone for now, in the spring I'll get some answers.
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744 & 745 GLE's (IPD a-sbars)16v M46, 245 SE(IPD a-sbars) auto, 242Ti M46(IPD springs and bars), 745GL 8v auto