Dragging brake calipers wouldn't cause the locker to bind, just keep you from diagnosing the issue. Resistance in the driveshaft at either the pinion or tcase would cause binding, but it's hard to tell if the brakes are stuck.
My working process is to ensure maintenance is done before diagnostics, like making sure everything is adjusted and lubricated correctly and nothing is bound up. From there, isolate things.
Once everything spins freely on jack stands you should be able to find the issue much easier. It very well could be the locker itself, I just don't want to blame the locker prematurely if it's actually upstream.

As for the 500 mile service, that would make sense if cherokees had locking hubs but they don't. pre-89 had central axle disconnect, but that doesn't disconnect the diff. It's spinning and ratcheting all the time going down the road, granted not under load so maybe that's why he said to drive it in 4wd before front locker service.

The only other thing that comes to mind when you describe that pop is tcase chain or splines slipping. Almost sound like the front output splines are not disengaging completely.
My next steps would be:
1. confirm your shift linkeage is adjusted correctly (takes 5 minutes) to make sure it's fully in 2wd when shifter is forward.
2. pull the front driveshaft and see how that affects the issue. If it still pops with the front driveshaft off, then your axle guy needs to warranty his work.

Ultimately, I'm saving up for an ARB for the front. I've had close calls with steering issues sending me sliding sideways down the highway at 60mph (amazingly didn't roll over) so I don't mess around with anything that can mess up highway handling.