My guess would be a leak in the Vacuum Hose Hard to say without more info though I had the exact same problem with my 1993 Jeep Wrangler Sahara. I took it in to the shop repeatedly and after replacing all the vacuum hoses they still could not figure it out. I finally took matters into my own hands (literally) and bought a hand vacuum pump to test the vacuum motor myself. When I took it off it was full of gunk and would not engage fully. I cleaned it thoroughly and it has worked perfectly ever since. I used the Chilton's Jeep book for guidance, but it's pretty straight forward if you are mechanically inclined and don't mind getting dirty. You'll also have to add some transfer case oil after you reinstall the vacuum motor.
If it is 89 to 95 it has a central axle disconnect, they go out all the time. Check that.
It should engage when you shift the transfer-case onto 4-hi,or lo. It is actuated by a vacuum valve. If it isn't locking in,then you have either a vacuum leak,the diaphram(on the axle housing) is bad,or the mechanism on transfer-case is defective.
Just shift the transfer case selector gear shift into the four wheel drive range of choice....hubs are engaged vis vacume control activated via a vacume switch located on the transfer case I think
See Recall ID # 05V460000
Depends on if you have the electronic or manual transfer case. The electronic ones, they advertise as "shift on the fly" capable, but this is a really bad idea. Either way, put the vehicle in neutral... if you have manual locking hubs, engage the parking brake and lock the hubs by rotating them clockwise to the "lock" position. Then, if you have the manual transfer case, you pull the transfer case lever either to the 4HI or 4LO position... if you have the electronic transfer case, push the button either for 4HI or 4LO.
They are right on top of the steering column. They push to the left to engage them.
Most four-wheel-drive vehicles are controlled by the transfer case. The transfer case has many years in it. A bad year in the transfer case will cause the vehicle not to engage.
The front axle shift motor will not engage if: A. No power / bag grounding or the more likely failure is B. No Vacuum (4wd will not engage unless vacuum line going to the module is working properly) check/replace vacuum hose
the front axle and transfer case for your Wrangler are vacuum actuated so you may have a vacuum leak, under the hood by your windshield washer solvent there is a black plastic cylinder with 2 vacuum hoses on top, one goes to the axle and one goes to the transfer case, one of these is most likely the problem, if the hoses are fine then you may have a bad actuator on the axle, good luck.
Put your trans in neutral and move the second ( 4wd) shifter into the desired 4wd mode.
check the wires to the transfer case, what year/model
If the button is not working, might be a vacuum line problem. Vacuum lines engage the 4x4, might need to check the lines to the 4x4 transfer case.