What the heck is up with 8-3/4" rearends?
I believe some year C bodies use the same rear as the E bodies.
Nope.
The reason you are finding them cheap may be because they really came out of a C body.
That's a possibility, because as you can see the '65-'69 C body rears are only a 1/4" different from the E-body 8 3/4's. But they really are different, I've measured both personally and can confirm this list is right in that case. The A100 actually does have the same width rear as the '65-'69 C bodies though. The E-body 8 3/4's are also less desirable though because the axles themselves step down in diameter after the splines, then step back up about half way down their length. What that means is that they can't be shortened, unless you make them REALLY short (they can't be shortened to A-body length). Most C-body axles had no such change in diameter along their length, so they can be shortened re-splined to A-body length. That's just the E-body axles, the housing can obviously be made any length you want, but then you're in for custom or aftermarket axles.
8 3/4 Housing widths, flange to flange
A BODY
'66-'72= 52 5/8"
B BODY
'62-'63= 53 1/4" (And '64 Max Wedge- tapered axles with center nut drums)
'64 = 55 5/8" (Exc. Max Wedge- tapered axles with center nut drums)
'65-'67= 54 1/4"
'68-'70= 54 15/16"
'71-'74= 57 7/8"
'71-'73 wagon= 59 7/16"
C BODY
'65-'69= 56 3/4"
'70-'71= 57 7/8" (Chrysler & Fury)
'70-'74= 59 7/16" (and '69 wagon)
IMPERIAL
'65-'66= 57"
'67-'69= 57 3/8"
'70-'71= 54 3/4"
'72-'73= 59 5/8"
E BODY
'70-'74= 56 31/64"
A100
'65-'70= 56 3/4"
D100
'65-'71= 58 5/16"
'72-'74= 59 7/16"