I have a 93 sub with 4 wheel drive. front drive shaft truns all

Does it shift to 4wd high range ? If so then shift to 2wd . What is going on here is the transfer case motor is stopping due to the motor position sensor has gone bad which means that you replace the transfer case motor assembly. But if you wish to make it a 2wd for awhile then remove motor and manually turn shaft . This can be done by raising and supporting rear axle truck in park remove motor turn shaft slowly with pliers and rotating the play in drive shaft. There are stamped markings on back of case showing gear positions..

most likely the front differential is defective,,, does park work on a hill with out e brake (does it roll?)

If you have locked hub drive in the front end then that shaft will be turning all the time when moving as the front wheels will be making it turn through the diff,. If you have free wheeling hubs that allow you to disengage the hubs then the shaft will be turned from engagement from the transfere case. In that case check that you are in fact changing out of 4 wheel drive and the actuator is not jammed or rusted in the engaged position

Sounds like the transfer case motor is bad.(located on transfercase beside front drive shaft yoke) 7"Long skinny motor outside transfer case.I had the same problem on my 93 S10 blazer , 4L60 E ( first year for the electronic transmission ) .changed it and is working fine now

If you have the expirience or know what I`m saying,it`s going to be ease,Put the front end of the car in jack stans,so the front wheel are free,take the tires off,disassemble the locking hubs or check them making sure they are locking right,if they are got to the transfercase you may have the problem there.

Jack up the rear wheels and put it on jack stands. Have someone get in it and put it in 4wd and drive. Release the brake. Rear wheels are spinning only right. Don't get under it but look under it and see if the front driveshaft is spinning. If the shaft is spinning but the wheels aren't it is likely a bad front axle actuator. If the front shaft isn't spinning it is a transfercase problem.

I would suspect the vacuum disconnect motor or the vacuum solenoids for the disconnect motor ( or the vacuum lines ), but best to confirm
Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

With the engine running ( and wheel chocked, parking brake on ) have someone shift into 4WD, and watch the shift motor to see if it operates
It is on the front axle, looks like this


If the disconnect motor does not shift, you can check the vacuum on the lines with a vacuum gauge. Blue line is 4WD operation, pink line is 2WD operation.
The vacuum solenoids are on the firewall, and look like this


This is the block diagram for the ESOF vacuum operation of the disconnect motor.


This is assuming you have checked the fuses for the transfercase shift motor, and the vacuum solenoids.

hi, try checking your transmission fluid first and you may want to change your transmission fluid filter for they might be clogged.

If Jeep is equip with push button 4x4 try this.
If you are stuck in four wheel drive disengage four wheel try backing it up 10ft to re aline transfercase gears.( Also linkage under truck maybe stuck.)

If Jeep is equip with lever on floor.
This one is a common problem lever on floor becomes stuck and won't let you disengage it. Have to Spray linkage with WD-40 or some kind of penetrating spray under truck where the linkage comes out of the floorand goes to the transfercase.

(Floor lever, Its tuff to get it to release I know I went through the same problem on my truck I had push it into 2wheel by using my leg.)

A very common problem is faulty tires or incorrect wheel balance of the front tires. Check for a bulge or a wear on the tires. If all is in fine find a mechanic to check your wheel bearings. Check for a broken strut assembly or shock absorber. Check for tire rod bushings and the balance rod. Check complete front suspension.