Rear differential compatibility B3 - B4

[Radek]

Hi Guys;
My son's 90Q broke its rear differential support - an aluminum casting part no. 893-599-131.  All this due, I suppose, to the permanently locked rear differential that we neglected to fix.
Our options are: 
1. get new support, fix diff lock, reassemble;
2. get new support, install new diff, reassemble;
3. possibly:  get the diff from a B4 that's at the local yard - but will it fit?  FA says no, but I recall seeing somewhere that the whole rear assembly (rear subframe and diff) can be transplanted from B4 to B3.
Is this true?  Any other options?
TIA.

Radek.