Don't mention the ZJ, it's a d44a with an aluminum center section, very problematic axle. The only thing the ZJ is good for is to pull the disc brake parts off for install on better axles.
You can't upgrade the front axle unless you're willing to go with different wheels, 5x5.5" is the smallest lug pattern a d44 hub will fit through.
The Rodeo rear d44 is the only d44 with rear discs I know of that's a decent swap into a jeep. Very plentiful in the junkyards.
A front d44 is well worth it, for the larger brakes, great histeer options, locking hubs, and serviceable bearings.