I had revived my old '91 Trooper after being parked for 3yrs. I've slowly been repairing as I go and was finally able to get the 4WD engaged after working on the clutch adjustments but at low RPM's the tranny sounds like it's trying to pop back out. Makes a grinding sound like when you try to force the gears in without the clutch. It's still engaged but what could be causing this?