Rive shaft have a heavy noise, sound like a fan, my que4stion is:, is it drive shaft or U joints?

the "backlash" felt by reversing the drive shaft direction is normally the clearance in the spider gears in the diff so to get rid off that ( not necessary unless it is excessive) is to pull the diff center out and fit new thrust washers behind the spider gears
next check the diff mount bushes as the diff reaction will move the bolts in the mounts and make the noise

A noise is pretty tough to diagnose via e-mail. It could be any of the u-joints, differential, output shaft bearIng/bushing(transmission). I like put the rear axle on jackstands and run the truck in drive at idle and then I use a piece of 1 inch heater hose about 3 feet long as a stethoscope. Put one end against your ear and the other in the area of the noise to pin point the offending part.

CK THE U JOINTS ON THE DRIVE SHSFT. YOU HAVE TO REMOVE THE SHAFT AND SEE IF THE JOINTS MOVE SMOOTHLY. IF NOT REPLACE THEM. ALSO CK THE CENTER SUPPORT BEARING IF IT HAS ONE. ALSO CK THE TRANS MOUNTAND THE FLUID LEVEL IN THE REAR END. HOPE THIS HELPS BILL

spin the drive shaft back an forth, if it has play then its your universal joints, also lift drive shaft up and down near the the bearing and see if there's play there as well.

Drive shaft joints? You need to run it on the lift and put a stethoscope on the housing near the axle bearings. They still may sound okey because they not loaded as on the ground. You could go for new u-joints and balance the drive shaft. Change the gear oil and additive if locking diff. Tape the rear tire diameter on both sides.

Grinding noise while driving ...if not when running but not moving..
Check :
: wheel bearings
: u-joints on the drive shafts
: 3rd members on the front and rear axles

If you can get your car on a lift, a mechanic can put the car in gear while it's on the lift and isolate where this is coming from.

The rear diff. turns the rear wheels. It is located in the middle of the rear axle. It won't last long if you ignore it. May just need dearing's but if it is driven the cog wheels will grind to dust or drop off this can cause the rear axle to seize up and can be a danger because the back wheels will lock up. If you are going to fix it yourself let me know and I will be able to guide you through the process

have the drive shaft U-joints checked for excessive wear. if they are ok the differential carrier bearings are worn and allowing to much gear backlash.

If not differential, ck the rear universal joint on drive shaft by rocking shaft back and fourth. ck for any excessive movement or noise.

jack the car up and take a bar lift up on tire and ck for play in wheel bearings . jeeps are bad about wheel bearings and ring & pinion noise.