Questions about rear axle swap ... disc brake ok ?

I've got a 92 Cherokee LIMITED ...

I am going to swap in a new ( junkyard ) rear axle assembly .

Is a axle with DISC brakes a direct bolt in ?? even if my current axle does NOT have disc brakes

Will WJ and ZJ axles work as a direct bolt in ??

No axle is a direct bolt in unless it came from a cherokee. And no XJ had disc brake from the factory. ZJ and WJ all have crappy axles anyways and the brackets are setup for a fourlink. If you want discs then get a XJ 8.25 and swap in ZJ brakes. Thats a solid setup for running up 33s -35s

some kj have the 8.25 with disk but those are wicked hard to find look there or put ZJ disk on an axle your running now like already stated above

The KJ(that's Liberty) axle is wider then the XJ axle. However, if you source a KJ axle, you could swap the brakes straight across to the XJ 8.25 axle. It has to be a 1st gen. Liberty though, because they changed to the 5on5 bolt pattern when they did the re-design.
Funny as it sounds, the closest thing to a direct bolt in, with the least amount of work, seems to be a Ford Explorer axle. You still have to weld in spring perches, but that's required for any axle you get(no stock Cherokee axle has discs, the D-35 has them on other jeep vehicles, but you still have to get spring perches)

Whatever axle you end up doing, when you go to rear discs, you need to swap the brake proportioning valve for a ZJ/TJ 4-wheel disc prop. valve, or modify the one in your jeep. There is also a residual pressure valve in the master cylinder that needs to be removed.