I haven't used the 4 wheel drive in years. I decided to test it today. I had the truck in neutral and just heard a bunch of grinding when I tried. Any idea on the cause of this. I haven't tried it with the truck moving yet cause I assumed it would be worse. The truck drives fine otherwise. Just won't go into 4 whel drive at all and makes loud grinding/banging noises when I try. The vacuum lines going to the front axle actuator seem fine.

 

Drive at a slow pace, and try and put it in 4wd. I think it actually is synchronized... so, it should work. According to theory, you can shift into 4x4 at up to 55mph.... Though I don't recommend that.

 

Don't put it in neutral keep it in drive. Then shift to 4WD. You can shift it when in drive stopped but may need to move a few feet for it to engage. If it doesn't you need to check out the vac lines to see if there is adequate vac to the cad.