So I just checked the CAD again. It is working fine and locking in 4wd. With passenger wheel jacked up, the drive shaft won't turn unless I turn the wheel by hand. Have also checked and adjusted linkage. Shifts smoothly into 4wd with no noise. Light comes on too. Everything seems good except the drive shaft not turning. Have also drained the t case to check for metal shavings. Nice and clean and stuck my finger in also to feel around. YJ has 117,00 miles and has been driven by a woman since 94. Prob only been in 4wd a handful of times during snowy conditions. Any ideas or suggestions on any other things I can check would be appreciated. I'm trying to avoid replacing t case until I'm confident I have checked everything else first.