97 190e 2.3 rear axel ratio vs. 91 190e ..
 

Does anyone know if these cars have the same differential axel?

I am thinking of swapping out the 87's for a used 91-

Haasman

What trannies are in each car?

Foolproof method: check the numbers. Get under the rear of the car, and look towards the front. Look on the lower-right corner of the differential (hint: don't look on the cover); there will be some numbers stamped into the metal. You may need to clean off some grime to read them. The final 3 numbers tell you what your ratio is. See if you can compare your current one to the '91.