Alright, here is the issue.

When I rev my engine up, while in gear, the vent changes to defrost and my 4X4 goes away. If I rev the engine while in neutral the vent doesn't change.

Could this be a short somewhere? Vacuum leak? Whats the best way to fix this? any ideas?

THANKS!

I have a 1991 XJ, I6, Automatic

 

Check your vacuum lines and make sure that black round black ball that is plugged into the vacuum system, on my 2000 the ball is inside the front fender i'm not sure about your year... when you rev an engine you are actually dropping vacuum and if that black ball which is the vacuum reserve is not plugged in you will lose all your vacuum for those few seconds of reving which i guess could cause you to disengage anything that is vacum engaged is using if that vacuum reserve is not helping your system