Nope.....
Just standard 11 x 2 1/2 drums- same as Civvy Dips.
The only REAL difference was a larger sway bar in front, and an additional sway bar at the rear....that's it as far as the suspension/brakes are concerned.

Check-
http://www.dippy.org

There has been some success with people swapping the rear from a later model Ferd Exploder into the M-bodies. This gets the full rear disc set-up, good parts availability, and a good selection of aftermarket gear ratios.

If you want to keep it all-Mopar you may want to look at later model Jeep Cherokees- I THINK (someone else will need to confirm) that they had rear discs as an option.

Chrysler just made such great drum setups they didn't see a need to switch!

Also- found this over at Allpar-

http://www.allpar.com/history/mopar/rear-discs.html

It uses aftermarket parts, but may be what you are looking for.

Good Luck with it, hope this answers your question...
CC.