Truck is 90 w250 with D60 front and NP205.
A few months ago I noticed that the front output flange nut had loosened and the flange had been banging around which in the process had caused the drive shaft ball joint to destroy itself and consequently when I drove in 4wd it made a horrible grinding noise which when I looked appeared to be a wobbling front shaft.

I ended up changing the seal and the front output bearing on the case, and getting the shaft rebuilt. When I was tightening the flange nut really tight with the case in 4wd, I noticed that it "jumped" a few times ie. skipped when I was tightening it.

Now, when I drive in 4wd hi, it makes a popping noise at the tcase whenever I am in a turn or let off the accelerator, and doesn't "rock" at the wheels like it usually would, making me suspect something in the case. It also sometimes jumps back to 2wd. In 4lo it works better, normal "rocking" from the tires and doesn't jump out of gear.

So, something's up with the 4hi gear in the case?

Front axle u-joints are decent.

Thoughts on what it might be and how much teardown I'd need to do to rectify it? Anything I can pull out from the front output with the case on the truck, or pull the case? Could it be the gears are not meshing properly, or are missing teeth?

Essentially 4wd "works" for getting me out of icy spots, over obstructions on the trail, but I'd question its reliability at highway speeds. Also I don't want the case to just blow leaving me without working 2wd (no problems in 2wd at the moment). The truck is at 485k kms and has maybe 1-2 years of road life left before a part out or a body swap.