First check that the "TCCS" fuse in panel at drivers footwell is installed and actually intact. Test it, don't just look at it
Removal of that fuse is a quick "fix" for dodgy transfer case or unevenly worn tyres creating problems.
I would normally suggest checking that the front prop-shaft is actually installed (removel being a more permanent solution to fuse removal) but as you say that 4Low is not working I doubt that would be your problem.
A failed Transfer Case control unit (the above TCCS fuse supplies it's power) may be on the cards or a wiring problem. Hard to tell. Any codes?