The front diff in the GV is a vacuum opperated unit and as a fail-safe
if there is no vacuum it will lock into 4WD, I would check for loose
and or broken lines that go to it.
Good Luck
Wild
Wh-wh-what?? Really?
Before I had my custom driveshaft made, I capped the rear output (paint-can cap + big hoseclamp) and tooled around in 4WD (so, front wheel drive). Unfortunately, I also had yet to re-attach my e-brake cables. When putting the truck in 'P'ark, with the motor running, all good. But, turn off the engine (and, with it, the vacuum pump).....wheeeee! Free rollin' downhill. Had to carry chocks with me and be creative.
That, along with everybody's bitching about the SOTF's susceptibility to vacuum-related failure on-trail and shenanigans require to defeat it, has me thinking otherwise.