this is an 01 2500 automatic. the truck will engage and disengage while driving with it in 4x4. it does have vacuum at the red line when in 2wd, and vacuum at black line when in 4wd. over 20 in mg. when applied. took the cad off the axle today and watched the fork move from one side of housing to other when i shifted into 4 hi. everything seems to be in good shape. how can i test out the vacuum motor to see if this is the cause? or does anyone else have any other suggestions? thank you. brandon

 

light does come on when 4x4 is engaged. was making a loud popping noise when engaging and disengaging.

 

You may want to get the truck off the ground , then see [ by some method of trying to hold ft drive shaft / some times you can find a place to pry with a piece of wood to rub against the shaft ] to see if it is in the transfer case or the axial , its not uncommon for some stuff to build up in the fork vacuum linkage , then using a vacuum gauge see how much your getting , then with a vacuum gun connected at on side plug the other , pump up with vacuum gun & see if there is a leak [ gauge goes down = diaphragm bad ] .