OK, I guess I should have asked this in my first reply, but you seemed to have already diagnosed it. Anyway, what exactly is the concern? What type of system do you have Manual shift of electronic shift? I'll assume you have electronic, if so, what happens when you push the 4hi button? does the 4 hi indicator flash and switch back to 2hi? And does the 4Lo work?

 

i have the electronic and when i press 4hi it flashes and then 4hi stays lit, the same happens for 4lo. the lights all work properly as if there were no problem at all.

 

OK good you're on the right track then with the vacuum problem. Here's what you need to do,
FIRST...check and make sure the vacuum supply line to the t-case is CONNECTED to a vacuum source. You have a 1996, so your vacuum resevior is actually mounted to the hood itself on the driver's side, make sure the vacuum hose is connected to the tank, and check for vacuum at the t-case switch with the engine running. It should have good manifold vacuum, if not then it is either disconnected or rotted.
Next, if you have good supply here check to see if it is pressent at the actuator (under battery tray) with the engine running and t-case ineither 4wd range. Based on a previous post, I'll assume you don't have vacuum at this point, if so, you have either missrouted the vacuum hoses at the switch, or you have a rotted vacuum hose. Come to think of it, with a 96 and a vacuum problem, you should look at the passenger's side of the transmission bell-housing for the lines rubbing against something, I have repaired several of these in that location, you'll probably find a rotted vacuum line in that location.....check there after you verify the vacuum tank is connected.