You need new engine mounts. My 94 I had did this very same problem. Apparently those mount are liquid filled?? but they wear out so when you put it in reverse the thing basically shorts out from the metal from the endine side of the mount hitting the frame. I know it sounds bogus, but after we put new mounts in mine, ran great. At first Ford said it was a sensor elswhere. They changed it and it didn't fix anything. SO after I paid 200 for getting that other work doen they said it was engine mounts and would do the labor for $150 to put in new mounts. I didn't want to pay high for price, so I got some elswhere and they put them in for me. Only after the got the car apart did they realize they had to drop the front end. Lucky for me I had the $150 quote in writing. ANYWAYS, I'd bet that is your problem.