If by rear end you mean differential, the answer is yes. Differentials from 1981-1985 W126s will swap into W123s. 2nd generation differentials in the 1986-1991 models won't fit.

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.