Burgie,
Lots of goods idea foregoing; I'd try the suggestion of disconnecting the vaccum servo and manually putting the axle in "high" and seeing how the truck drives, if for no other reason to see if the axle itself is ok.
Related story regarding noises vs. point of origin: about 15 years ago, I was working my way through college, and had a summer job as a laborer for a mason. I got to drive a '75 or '76 Chevy C-60 flat-bed w/ a four speed tranny & 2 speed rear. The axle was shifted by a knob and cable which came up through the floor bewteen the driver's door and the seat (possibly an aftermarket jury-rig); when changing axle speeds, if it didn't completely engage, there would be a horrific clashing/grinding noise from under the truck. It seemed like it was "under the cab", but that didn't make sense to me, as the gears that weren't meshing were way back in the diff (it was a 12' wb truck or longer). I finally figured out that the gear grinding was being "telegraphed" up through the drive shaft and through sloppy u-joints and splines, there was a lot of stuff along the way to make noise.
This truck was pretty beat though.
Hope your case is something simple ( and inexpensive !).
