I read something before about the old W124 4-matic before, the rear differential and center differential on 4-matic can be automatically locked according to the speed and slippage.
So I don't think 4-matic's rear locking differential is interchangeable with the one from RWD 300E.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.