Yes, there is a GM synchromesh additive that is added to transmissions such as Muncies. The limited slip additive is a friction modifier and works to keep the cones or clutch plates in the diff from grabbing and chattering. If you go around a corner and your diff sounds like it's about to fall apart some additive may be all you need. Same goes for synchromesh transmissions, modifying the friction and allowing for smoother operation of the syncro cone/clutch. Some transmissions, such as the Tremec, just use synchromsh fluid and no 80/90. The fluid is quite thin compared to the 80/90 and can cause a lot of clacking in the transmission at idle with a large camshaft, which may cause alarm with an owner thinking his/her transmission is in a sad state. Been there/done that.
I think the limited slip additive and the synchro additive are close enough to be interchangable, such as stated on the bottle/can you have.