Because i've attempted to put it in and it will not engage. I've pulled the shifter back as far as it will go and popped the clutch and only the back tires spin. it had worked fine untill i put a frame underneath it.
What year is your Jeep? The Command Trak system on the front axle that runs the 4X4 changed over a little somewhere around 1990. It is usually a fairly simple fix.
First, to see if you have proper vacuum. Does the light on the dash (4X4) come on when you engage the four-wheel drive? If so, depending on your year model, that should mean you have at least some vacuum. Start there, and report back.
If you are feeling froggy, pull the vacuum harness off the Command Trak and feel for vacuum or better yet borrow a tester from Autozone. If you are getting proper vacuum, though, you should be able to feel the vacuum pull on the harness without a tester--just hold a finger over the holes. Be sure to cover the hole that runs to the vacuum line that controls the dash light or you will have introduced a leak---it just takes three hands ;-).
Be very careful testing the vacuum at the harness with the jeep idling---you don't have to put the jeep in gear. DON'T RUN OVER YOURSELF. Seriously.
So if you are saying that it won't even shift into those modes (the shifter itself won't budge), then I am not sure what your problem is. If you are saying that it doesn't work when you shift into those modes, I would check vacuum.
So can you physically shift to those positions or will the shifter not go that far? Have you checked underneath to make sure the linkage between the shifter and the transfer case isn't hitting anything or obstructed?
The rest of you correct me if I'm wrong...the '95 should still have the NP231J with a shift pattern of 2h-4hi-N-4lo starting from forward working back. You can shift the first two positions which should put it into 4hi. You have to physically move the shifter to the right and pull down to get it into N and then 4lo. I know when I picked mine up a couple of months ago it was a bear to get it to move to the right because it really hadn't been used for a number of years. If it is a matter of not shifting you can have a linkage bind or it could just be stiff like mine was. I recently replaced my carb with a Weber and disco'd a bunch of vacuum hoses and inadvertently moved the vac hose going the axle so when I went to use my 4wd the other day it didn't work. Put it up on jackstands and put it into gear to verify that the front wasn't locking and then tracked down the hose...all is well now. Locate the red vacuum line on the passenger side of the engine and make sure that it is hooked up to manifold vacuum and make sure that the line at the axle(yellow, I think) is hooked up. Follow it and make sure that it is not cracked or disconnected.
I did a little more digging...that shift pattern is from the 80's CJ's. Either somebody replaced the knob because the old one fell off or you're running a Dana 300 t/c that somebody swapped in. I don't know the compatibility with a '95 engine but I'm sure somebody here can tell you what to look for other than physically checking the tag on the t/c to make sure which you have.