[lurch115]

to check the linkage have someone sitting in the truck and have them move the lever from 4 high to 4 low. WITH the vehicle shut off and their foot on the brake with the transmission in neutral. i resently had a similar issue with an NP203 which is the chain drive version of your t-case and come to find out the shifter was actually the problem. also have to ask, as i know it can be easy to over look things. when you shift in to 4 Low have you tried pushing down on the shifter and then pushed it ahead more? i've seen the shifters "freeze up" before and not want to push down. in that case i usually take a dead blow hammer and give the shifter a few light taps straight down and that usually frees them up.