If you have verified that the transfer case itself is engaging into 4wd, then this is where I would begin my diagnosis.
On the front axle, on the passenger side, you have a vacuum actuator that moves a fork in your front axle. The should have 2 vacuum lines running to it, one red and one black.
You will want to verify that you have vacuum on one line while in 2wd and no vacuum on other line, with engine running.
IF you have vacuum on one hose in 2wd shift to 4wd and verify you have vacuum on other hose, and none on other. If vacuum is operating as supposed to, you will want to remove the actuator cover from the axle and verify that the fork is sliding on the actuator and is not binding up. It is very common problem on these for the actuator rod or fork to bend, causing binding and lack of 4wd.
If vacuum is not present, trace the lines from the front axle to the transfer case verify all lines are hooked up and not damaged. They will attach on the transfer case at a 4 way vacuum connector on the top driver side of transfer case.
If these lines are okay, you will want to verify you are getting vacuum from the engine to the transfer case. If not follow the line up to the engine compartment and find broken or damaged line.
If you have vacuum at the transfer case, but nothing coming through, the vacuum selector switch in the transfer case is bad and will need to be replaced.
Make sure you have vacuum. This is critical. It should be whatever engine vacuum is. IF it is weak, it may not be enough to move the actuator and can also be caused by damaged or kinked lines.
Hopefully this information is helpful. Thanks and good luck!