1991 cherokee transfer case problem

i am having a problem with my transfer case on my 1991 cherokee laredo 4.0 litre inline 6. i used to have no problem throwin it into 4H when it would start snowing. i could easily plow through 2 feet. Just recently last winter, when i would try to put it in 4H it wouldn't go in, or it would a little while after. If it does kick in, when i go up the slightest incline it will slip into 2H causing me to fishtail a tad. every time i pull the lever to transfer, it goes in smoothly, but never engages. It goes into 4L absolutely perfectly and i can just feel the pull of all 4. Any answers i would greatly appreciate it.
thanks

If yours is vacuum actuated, you might check that.

Some front axles through 91 are vacuum disconnect, 92+ are non-disconnect.

So I should check the actuator and the vacuum lines to the front differential? Any tips on how I would know what is actually at fault? I just don't understand how 4L would work fine while 4H won't even engage.

Try also checking the shift linkage for the transfer case as since it works in
2H and 4L but not 4H it might be that when it's moved all the way to one end
or the other it will stay there and engage but when it's in the middle for 4H is when it won't.

Yes, look at the shift lever, there is also vacuum control there too. That is where it tells the front axle that you are shifting into four wheel drive.

For the front axle just in case you need it:
http://jeep.larrynova.com/88/88_Transfer_case_command_trac_with_vacuum.pdf

I had my girlfriend shift her into 4H and 4L numerous times yesterday while i was looking at the transfer case. it came to my attention that when engaged into both the front drive shaft spun when i told her to move the car and when not engaged it wasn't moving...seeeeems to be working...wierd. In both cases, the "part time" light on the dash did not come on when in fact the front drive shaft was spinning. Any help? I am going to take her off-road to see of she still slips in and out in 4H like she did in the snow. I don't like the fact that the light doesn't come on. I'm thinking its just a sensor. Can someone tell me where it is if this is in fact the problem? I'm thinking its an easy fix. Thanks for the help guys, I appreciate it.

The front vacuum actuator is what connects that spinning drive shaft to the front axles.

The light switch is there also. See my link above.

So it sounds to me that this is most likely the problem. The functional test on the link you sent me seems to be a little confusing. Do you know if it is it difficult to do yourself and do I need any special tools out of the norms? Thanks alohabra, you've been a huge help.

The vacuum pump is a hand pump.

When I was at the last Jeep Jamboree there was a Wrangler with this problem. They fixed it at a local shop/gas station place. They connected the front actuator directly to vacuum so the front axles were engage all of the time. The Wrangler was able to finish the Jeep Jamboree with 4 wheel drive working (which was definitely needed for the trails we were on).