Hey, I have a 2001 ford ranger xlt extended cab 4x4 myself,
unfortunatly there are a number of things that might have gone wrong.
You know the electric switch 4x4 system won't go to 4hi unless you
under 45 mph, it could be part of the electronics that detects that,
and 4low only works if you stopped, in park with your foot on the
brake, so it could be any of those sensors. Your 4x4 mode will only
show on the dash light if it succesfully engages 4x4 mode, so it could
be the vacuum motor that engages your hubs, or part of the transfer
case also. check the vacuum motor, check every fuse in the dash, and
also the fuse boxes under the hood as well. Allot of people found the
vacuum motor has gone out.