1991 jeep wrangler front drive shaft isnt turning. Once in a while gears will grind but front diff isnt working

You only have one option. (well 2, if you don't want to drive the truck) the front diff turns no matter what you do. Your pinion bearings are shot. The pinion will still turn when you drive it, even if the driver shaft is gone. Bearing and seal kits run around $300. For newer models. Older ones a bit more. Once the pinion and ring gear go away, add $200. Spider gears and such. Costs go up to as much as $1300. You can get a used diff for around $300. Depends on year make and model.

old dog , jeep, sure.
its a PART TIME 4wd, not AWD.
you post but forget (100s like this here) with no road surface stated.
dirt, ice, snow, off road, pavement. what??

my wild guess, is this car is new, to you,
and you never read the operators guide yet.
lost 25 years ago, but sold on fleabay
PT-4wD
do not drive on pavement dry , or you WILL wind it up.
WHEN I SAY 4wd I MEAN , the front lockers are locked AND
the transfer case is in 4wd.

Here is the best page on the net for PT-4WD
here we go , WINDUP. 101

http://www.rubicon-trail.com/4WD101/driveline-bind.html

answer, is , DONT DO That.

If the external linkage is attached and working it pretty much has to work unless it has an internal failure. (Usually pretty noisy when that happens)
Many times I've seen a differential failure be mistaken for a transfer case problem. disconnect the driveshaft from the front diff and see if you can turn the yoke with the vehicle on the ground. If you can, you should remove the cover and take a look at the gears. Also, if there is a control unit on the passenger front axle, make sure that is working as well.

if the cv shafts are spinning then the front diff is working, sounds like the front hubs need to be inspected

If the shaft is spinning thn theres nothing worng with the diff...(if i read right what you explained), the problem seems to be the drive shaft,probabely the splines at the front hud is gone or maybe the swivel joint has given up.

if this is a 5 speed manual transmission...all you need to do before putting your jeep into reverese is puting it into first gear and then shifting it to reverse to avoid any grinding or problems shifting into reverse...this isn't a problem just a poor jeep design by not installing an extra gear to eliminate this grinding or shifting problem

Our Jeep Wrangler YJ's are equipped with a vacuum module on the front axle. It is used to engage the front axle when the transfercase is shifted into 4WD. Check the vacuum lines on the front axle to make sure they are connected. The passenger side axle shaft has a disconnect sleeve that is moved left and right by the vacuum.

Your clutch is not disengaging. If it a hydrolic clutch chk. the fluid in resavor, Or slave cyl. for the clutch is bad, Or it has got air in it and needs to be bleed. This is a clutch problem, nothing wrong with trans. If its a manual clutch linkage its an ajustment thing. My best guess if its hydrolic slave cyl. went bad, or resavor is out of fluid.

It sounds like your driveline to the rear wheels is broken. Could be transfer case output shaft to the rear wheels has broken. Try disconnecting the rear drive shaft and see if you can turn it by hand with the transfer case engaged in 2WD. If you can turn it while the Sahara is in gear, the output shaft is no longer connected to the engine for some reason.

transfer case or front diff to check put transfer case into 4x4 go under the truck and move the front drive shaft it should go in if working. Hope this helps.