Want to add disc brakes and a locker to my '04 Chevy Tracker. Are there any other rear axles (Jeep, Ford, etc) that are good candidates for such a swap?

First question - can we assume your Tracker is 4WD?  If yes, are you planning to swap the front axle also?
Second question - what are your fabrication skills like - or - do you have a fabricator you can pay to do the task?

Have you figure out where I'm headed yet?  The answer to your question is determined by what YOU can do, or can afford to pay someone to do.

The only other bolt-in axle is another Tracker/Suzuki axle - there are "rear disk" conversions, and lockers that can be fitted - search the internets - ARB air lockers work

Yes, it is 4WD.

No, I will not be swapping the front diff. I have manual front hubs, so I will be "Lincoln locking" the front diff.

I installed the Altered Ego 4.5 inch lift myself. I have, in the past, rebuilt engines, auto transmissions, etc. So I would be doing the work myself.

My question is, based on the experiences of those in this forum, what is the best option (year, make, model of rear axle) for me to use in order to most easily accomplish my goal of adding a locker and rear disc brakes?

Well - I think you missed where I was pointing you - so I'll come outright & say it - axle gears - front & rear axle gears must match, or you'll end up with drive train wind up and break stuff (like the transfer case) - iIf you change the reax axle, but not the front, you'll need to find an axle with gears that match the front, or that you can get a matching set of gears for.

Fabrication skills - I'm not talking about bolting up someone else's kit, or wrenching on an engine - I'm talking about building your own suspension links, cutting heavy steel plate, welding it to the axle, that sort of stuff.  Swapping an axle from another make & model is not going to be a bolt-in affair, the chances of your finding a five linked live axle with the mounts in exactly the right spot and at the correct angle is slim to non-existent, so you're either going to be fabricating mounts and welding them to the axle of your choice to accept the existing links, or fabricating new links & mounts and welding those to the frame.  Welding new brackets to the axle is probably easier, but is not for the "inexperienced", not only because failed welds puts your life at risk, but there is a risk of warping the axle.

At this point you have "two strikes against you", or rather, your plan - you might be able to find an axle with suitable gearing (I've got two different 4WDs in my driveway that both have 4.875:1 gears, so it's not impossible), but finding one with the right gears AND with all the mounting hardware in the right places, I would say, you're hunting for the holy grail (going back to my two 4WDs, both are live axle, five link rear ends, but the mounting brackets are not even remotely close).

Going back to your end goal - and the last statement I made in my previous response - the only other bolt-in axle is another Tracker/Suzuki axle - there are "rear disk" conversions, and lockers that can be fitted.

Regarding that proposed "Lincoln locker" in the front - you don't say which Tracker you have - most of them have alloy front axle housings, if that's what you have - plan on replacing it -it WILL break.