Recently I replaced ball joints, right outer axle shaft and u-joint, shift fork and collar and put in a Posi-loc cable system. Tonight I took my truck into an old gravel pit to test the 4wd system. All seemed fine until I turn the wheel, then a really disturbing metal popping sound comes from under the truck in the front axle area. Like splines seriously jumping. Like things are getting damaged, bad.

My truck is a 2001 3500. It has some front differential ring gear backlash and the shaft into the differential has slop at the differential end, pinion gear and shim need attention. What it seems like to me is that the shift collar isn't engaging fully onto the intermediate shaft, and popping. Thats what I hoped it was, so I re-adjusted the posi-loc cable to pull the collar and fork further, but it still does it. Can't tell where the noise is coming from but it only does it when turning the wheel. On gravel the outer wheel is traveling further and shouldn't the differential slip?

I'm hoping its not the front differential as that is beyond my mechanical confidence. Has anyone else had this problem? Fluid level is fine with top grade synthetic 75W140.