If there is any difference in diameter between your real tyres and the small spare, then the small spare should only be fitted on the front. The limited slip diff is too tight to run uneven diameter tyres on the rear.
Also, the small spare wheel is deliberately set up out of balance, so that you can only drive at around 15 - 20 MPH. This is a safety thing, so that you stay within the limits of the smaller tyre.

I would suggest you have damaged the rear suspension and/or differential. If you take the vehicle to a good mechanic together with these comments, they should be able to find the damage and repair it.


PK