Are you sure you have the right kind of axles for the diff? IIRC there is about 4 different ones.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

There are many different ones. This is why the spacer shim from the right side may be a different thickness from the left.

Machining tolerance varies such that the length of the inner stub to the clip groove is different. The correct spacer (shim) sets this to the specific differential.

If you swap axles, chances are you will need different shims.

Still, this does not help solve zsmith29's problem of the outer splines not meshing.