1990 F150 Reverse in 4WD Problem
 

I just purchased a well used 1990 F150 and I have a problem. When the truck is in 4WD high, reverse doesn't work. It sounds like I'm trying to shift it without pushing the clutch in. It's a 4.9 liter engine with a 5 speed manual transmission. Reverse works fine in 2WD or 4WD low. Any ideas?