Sounds like your transfercase linkage need to be adjusted. If it is off a bit then you will not be able to shift the TC into the mode at either end of the shift lever push/pull. To check it out do this:
1) Go under the Jeep and find the shift linkage that is attached to the transfercase. On the rod part of the linkage is a nut you must locate.
2) Loosen this nut and move it around on the shaft until you can tighten in and climb back into the Jeep and get shifted into 4low.
3) Once the TC 4low loosen the nut again. Climb back into the Jeep and position the shift lever about a 1/16" to 1/8" from the bottom 2low shift setting.
4) Go back under the Jeep and tighten the nut real good.
5) Climb into Jeep and Verify shifting for all TC modes.

Also check the nut on the shift linkage where it attaches to the TC. This could be loose too. Good luck getting er' done!