There is a shift fork that is engaged using vacuum from the motor. The shift fork is located inside the front axle more towards the passenger side. When you engage four wheel drive it lets the vacuum go down to a motor that moves the fork to lock the hubs
There could be many reasons, but you should start with this simple test that requires two people. 1. Start the vehicle. 2. Set the emergency brake and put the transmission in PARK (for automatics) or NEUTRAL (for manuals). 3. Remove the vacuum plugs from the shift motor. You should be able to feel vacuum suction from one of the holes. 4. Put the transfer case into 4WD. The vacuum suction should switch to the other hole. If it doesn't, then you have a vacuum leak somewhere. Check the lines for a leak. If suction does switch to the other hole, then the problem lies within the shift motor. The shift motor is a *very* simple mechanism so it's unlikely some is broken inside. More than likely it will be "gummed up" from old oil or any number of reasons. Remove it (4 small bolts) and clean it up, but before you re-install it, connect the vacuum lines to it and shift the transfer case in and out of 4WD to see that the forks slide left and right. www.wrangleryj.com is a site dedicated only to the 1987-95 Jeep Wrangler YJ.
Stick shift.
It could be the shift motor on the T Case. Or it could be the automatic shift button switch on the dash.
Engage the clutch. Shift to neutral. In neutral move the smaller shift lever. On an 87 wrangler the lever is closest to driver. The shift pattern is 2H at the top, 4H middle and 4L is all the way down.
automatic make sure all grounds are attached and the plug is tightly pluged in
How to peplace A RANGER transfer case SHIFT MOTOR
Your synchro is going bad.
You may have to have your clutch adjusted,
If a 2000 Jeep Wrangler suddenly loses power in 5th gear, it might be a problem with the speed sensor on the transmission. This sensor regulates when the Jeep will shift or should shift.
Underneath the the plastic cover on the front differential.
Where is the fuse or relay for 2000 f250 4x4 shift motor