4wd does not work at N O T

So, as long as my engine is cold, my 4wd works just fine. When the engine is up to temp it will not engage. The truck is a 95, its vacuum, I have not done any of the testing recommended by the haynes manual. Just hoping someone else has had the same issue and can tell me what might be the problem.
A Question: Will an electronic unit from a newer Tacoma swap in to my differential? Is this a good or bad idea?

 

Does the light not come on or can you not hear the 4x4 ingage.

With my 96 sometimes the light wont pop on so i thought it wasnt working. come to findout it does just the light is shorted or something.

 

The transfer case is actuated electronically, but I believe the automatic front hubs work on vacuum. If they do, there's probably a check valve and/or vacuum reservoir somewhere in the system. One of those things being faulty would make sense in your situation. When you're at WOT, you're not making vacuum.