I've got a little more to add to this.
I'm a Mopar fanatic so I'm very familiar with the 8 3/4" rear axle but I'll admit to not knowing much about earlier rear ends that Chrysler used. I know they say the 8 3/4 was introduced in '57 but as everyone mentioned, Panzers were using something in the earlier '50s that looks a heck of a lot like the 8 3/4. I sent an email to Richard Ehrenberg, S.A.E., the tech editor of Mopar Action magazine asking the following: "Hi Rick,I have a general question about rear axle assemblies used by Chrysler before the 8 3/4. There doesn't seem to be much info on the web regarding '50s rears. I'm coming at this as a fan of Panzer garden tractors which used narrowed Chrysler rearends. From what I understand, the 8 3/4 was introduced in '57? These tractors were produced starting in '54 yet to my eyes the rear axle assemblies appear the same as the later ones. Just how far back did this design go and what makes the 8 3/4 that much different that they say it wasn't introduced until '57?"
His response (in just a few hours... on a Sunday no less): "Jim- The pre-'57 axles were similar in many ways, in fact, most (but not all) used an 8.75" ring gear. The primary visual difference is in the differential, most (but, again, not all) versions used a two-piece, bolt-together case, similar to the '58-'68 Power-Lock Sure Grips. There were several design changes between '56 and '57, enough so that, for example, the carrier, and even the carrier-to-housing gasket is different, and few hard parts interchange, not even the ring gear bolts. If you press me to be more specific, however, I'd throw up my hands in frustration -- I simply don't
So basically, the 8 3/4 rearend wasn't really introduced in '57. The basic design goes much further back. They just made enough upgrades in '57 to call it a new rearend.