Replaced the locking hubs with the new ones from Mile Marker. Fit like a glove and seem to lock as they should. I went to a neighbors house with long uphill gravel driveway to test out the 4WD. Def seems to be an improvement, didn't hear much popping sounds except at what seemed like a regular interval while going straight at low speed. Couldn't differentiate between sides, which made me wonder if it could be the transfer case chain. I read a few posts / watched videos about how the chain inside the transfer case becomes loose on these older models and slips. Sounded like that, reminiscent of the early days of riding a bicycle and switching gears / ie; chain slipping a bit whilst jumping to the next sprocket - but amplified obviously.
How likely is it that could be the/an underlying issue?
I doubt its the chain, under load it would be under tension so it shouldn't be slapping the case. Transfer case chains usually will stretch a little bit, but if it has stretched enough to be hitting the case someone at some point was really beating on that truck to cause it to stretch far enough to hit the case. I don't think there's really any way to check without tearing the case apart, possibly if you have one of those little scope type cameras you could stick the camera down in the fill plug opening and have a quick look, but you probably couldn't see anything unless you drain the fluid out of it of course...might be worth the effort and possibly borrowing or renting that camera.
Most likely you still have a U-Joint issue somewhere if you are still getting a popping noise from somewhere. Maybe grease all the U-Joints and see if the noise goes away, could just be a dry joint in there somewhere. I believe there's only outboard u-joints on the front axle, possibly 1 or 2 on the front drive shaft, and 1 or 2 on the rear drive shaft depending on which setup your truck has. I had the drive shafts in my Bronco 2 rebuilt with U-Joints, expensive but much easier to service. If you still have those odd ball CV type joints in your drive shafts it could very well be one of those joints is failing causing the popping noises as the axles flex and move with the terrain.
Maybe jack the truck up front and rear and give the wheels a spin and see if you get any noises that way too, might be easier to pinpoint the noise if it happens to do it on jack stands in the driveway with the vehicle shut off.
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