My fiancee owns a 1999 Dodge Ram 2500 Cummins diesel. He tries to put it into 4wd and it wants to jump in and out.
He did trying blowing the vacuum lines out and that seemed to make it a little better. He says there is some kind of gear lube in it.
Is there a diaphram maybe causing this to happen? That is what he thinks it is, wanted some input before he tears it down.

 

Sounds like the vac pump is out.

 

He seems to think; there is a diaphram that has been damaged and it is letting gear lube get into the vacuum lines. w/the lube in the lines it isn't getting good enough vacuum. He said tonite, that when he gets the rpms up it will go into 4wd, but as soon as rpms go down it pops out of 4wd.
Thinks the vac pump is okay.

 

Well it weak, so he needs to replace it either way.

 

Possibly a faulty switch on the t/case that could be letting the oil in the lines and not letting enough vacuum through the lines. The switch just has a ball that sets on the linkage inside the T/case that goes up and down with either 4wd or 2wd so it could be leaking and sucking gear oil up into the vac lines and causing your problem.

 

Updates??