Transfer case probs

I have a 89 xj with the np231 t/c and aw4 trans. I had posted a couple of weeks ago about a grinding when shifting from n to 4hi. The problem I seem to have now is I can not shift the t/c unless I shift the transmission to neutral, which I know is bad because it makes some very unfriendly noises when this happens. I am new to the 4wd "scene" but not to cars. Is the problem within the case itself or could it be in the linkage?
Any info anyone can provide will be appreciated
Scott

I had an 87 that would make a terrible ratcheting sound sometimes when shifting into 4X. Turned out to be the shift motor in the front axle, I´d have to periodically spray some oil into the vacume inlets (things got sticky). Removing the vacume lines form the shift motor, can cause problems got to be carefully, wiggle, twist and pull with feeling or something is gonna snap or break.
They discontinued the shift motor on the right axle tube housing, sometime around 90, but I´m pretty sure yours still has one.