My 92 jeep wrangler, when in $ wheel drive, only spins 3 wheels. the 4th wheel never engaged, i figured it might be a limited slip thing, but I had it free spinning for awhile, and I would have thought the drivers side front wheel would start to spin at some point. any suggestions? answers?

what wheel spins, double posted with this one add small fact.
front or rear.???? spins, gee, must i ask this?
you have open axles, right>? and this is normal
it is !
unless you use Lim.S.D. Axles (an option new)
or drag your brakes, creating a virtual Lim_S_D.
Limited slip differentials. (closed axles)
my 2008 wrangler does that, via software and uses ABS to lock a spinning wheel. (drag it)
but i cant drive on ice free pavement, are you, if yes, stop
PT 4wd is not for that. at all
you need full time,or AWD to do that magic.

got open axles? goggle that.

IF you have both front wheels in the air then either wheel will spin freely unless you have diff lock or limited slip diff. That is the nature of a diff..wd means that all 4 wheels drive but only when you have a limited slip diff or a diff locker. in action . If you have both of one side wheels in a bog you will not go any where.

Don't have your foot into the gas when shifting into 4 wheel. Best is to shift at a lower speed and "feel" the shifter as it goes in. Abandon the attempt if it grinds (picture parts doing really bad things to each other)
One general driving tip...Never do anything on ice. Shifting, steering, braking or anything else can throw you out of control on icy surfaces. Anticipate bad conditions and begin using 4x4 before you are in a situation where you need it.
Practice shifting your unit...though they are "supposed" to all work the same, every one I've ever driven had a certain amount of "quirks" that had to be learned.
About the one wheel spinning...most people don't realize that on most jeeps you have tow "open" differentials which means that at any one time only two wheels are driving you (except those with limited slip rear diff, in which case you have 3.)
Good luck with it

If you have an actuator box on the passenger front axle that may be disconnected or broken. If not remove the cover from the front differential and check the gears...they may be broken.

Best way to attain engagement without any grinding etc. is to pull 4x4 lever into gear while rolling at low speed. You can also in an emergency, turn the engine off and engage, then re-start (when you have been in 2wd and have gotten stuck in mud and can't roll) When dis-engaging, generally it will shift out with no problem while moving, but if it does not, then you can back up about twenty feet and usually that will work. (still beats getting out and doing it manually at the hubs!!)

4 wheel drive is vacuum activated. Check vac motor 1st as it is probably the reason for it not engaging. I had a 88 cherokee w/ bad vac motor which caused exact problem you describe.

Has this Jeep been modified at all?
It sounds like the shifter cable was reinstalled backwards.
Specifically the 4-Low "Not Climbing Well". 4-Low should give you the best torque and hill climbing ability.
Do you feel any "Binding" in the steering wheel (difficult to turn)?
Remember a Jeep uses real driveshafts and differentials to give you 4WD - you cannot engage 4WD or 4-Low on dry hard surfaces. It needs a little bit of slip between the wheels for turns.
Check you're fluids too.

Check linkage adjustment and that it's engaging fully. if that's OK, problem is inside transfer case.

usualy you will only have two tires spin at a time even in 4 lo or 4high. those tires are going to be diaginal from each other tipicly. now since your vehcle is equiped with quad drive it may employ a limited slip rear differential to help it along. however this only afects the rear axle. now you can test it by jacking your vehicle up and setting it on jack stands. one for each wheel onder the axle tube on a good solid (concreat or black top) flat surface. then have some one you know and trust to get in the car and put it into drive . now you can actualy see what wheels are spinning or not.now is were the limited slip comes in if you have one rear wheel spinning and the other is sitting still the limited slip will engage and start the other wheel spinning . now this may bang until the other wheel catches up (note that the faster the wheel is spinning the harder it is on your truck)now be very carfull stay away from the wheels tires and all other spinning parts it is a machine it can hurt you badly. but give it a try and it will answer your question. be safe hope this helps

Check your fluid levels, you need to verify that the transfer case has fluid in it. Also, on one of my old jeeps the lever that put it into 4 wheel drive just stuck a litle. After yanking the **** out of it and moving it back and forth, drive a litle then stop and do it all over again.. a few times. it went into 4 wheel drive. this is why you are suposed to engage your 4 wheel drive once a month and drive around the block, just to make sure it gets flid where its supose to. if it doesn't get engaged then the fluid just sits at the bottum of the transfer case and doesn't get up on the gears.