I'd bet that it's the accumulators (there are two...one each side). Both my brother-in-law and I have '88 300TEs, and a short while ago his accumulators went. Car bounced like a pogo stick. Because he couldn't get a straight answer for a while, he put off the repair. Bad move, because all that suspension movement appeared to have prematurely aged a couple of rear suspension pieces that required replacement too. The repair cost wasn't too bad on the accumulators, so I'd jump on this right away.

It's highly unlikely that your rear hydro struts are bad, by the way, especially both of them. My mechanic's never seen a 300TE strut go bad, and mine are fine at 145k miles as are my bro-in-laws at 260k. Good luck!

Michael
'92 500E
'88 300TE

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