I wish I could be of more specific help, but I used to own a 1990 300CE that had the same problem. It wouldn't go into 4th gear (come on people, give the guy some credit and assume he knows when his car isn't going into 4th gear ), and it turned out to be some sensor type thing installed on the transmission that the dealer was able to replace and resolve the problem. For some reason I want to say it was a component other than a vacuum soleniod. I sold the car and gave the records to the new owner, so I can't give you more info than that, but my point is, don't let some shop tell you that a new tranny is in store. Take it to a good shop that can diagnose the problem and fix it properly.

Greg