Was just browsing through my Haynes and it said that either the linkage/t-case is damaged or there is not enough oil in the t-case? Have you checked your levels yet?
When mine was not engaging the shift fork slot properly, it was very hard to shift, like it was binding. It would sometimes shift and sometimes not, leaving the transfer in one position or the other. It felt like the "L" shaped arm, pictured previously in this thread was scraping on the bottom of the shift rail. Every so often it would catch the slot and shift it, but if the case was a little bound up or something, then it wouldn't shift. Either way, the feeling of being hard to shift, like it was stuck was something I experienced.
It is very easy to get to the arm on top of the case from behind the case. Put the shifter in the location that you are currently in, so if it is stuck in 4 low, put the shifter in the 4 low place, then lift the arm with a screw driver or small prybar, I bet it will raise up a bit, like 1/8 inch or so. That is the arm going into the slot. Have someone shift from 4 low to 4 high, then try without lifting it, I'm sure will see that this is the issue.