4 wheel driver not working

Someone will have to go under and check the linkage is working when the 4 wheel drive is fully engaged and not just moved to a neutral position.

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

Has the transfer case ever been serviced? The fluid should be changed every 30,000 miles, if not the works can stick and cause engagement problems. Make sure the vacuum to the front hubs is available, if the front hubs are not locking the front drive shaft will turn but no Front wheel lock up and no 4 wheel drive is available.

SOUNDS LIKE A BAD HUB TRY THIS TO DIAGNOSE THE PROBLEM

Put the transmission in park, set the parking brake and raise the chassis so one of the front wheels is off the ground while the other is still on the ground. When the transfer case is in two-wheel drive, the hubs should be released allowing the wheels to turn freely when spun by hand. If the axle shaft turns when you spin the wheel, it means the hub has not disengaged.
To check engagement, rotate the axle shaft backwards. This should lock the hub. Try turning the wheel by hand again. The axle shaft should now turn with the wheel if the hub is locked. If the hub fails to lock, the hub will have to be disassembled and inspected or replaced.
To check release, hold the axle shaft steady and rotate the wheel backwards. You should hear a click as the locking mechanism slides back out and disengages the hub. Rotate the wheel forward again and it should turn freely.
Another way to check the hubs is to raise the vehicle on a frame contact lift so all four wheels are off the ground. Then start the engine and place the transfer case in four-wheel drive and the transmission in drive. If the front wheels don't turn, look at the front axle shafts. If both shafts are turning, but one wheel is not, the problem is a bad hub on the wheel that isn't turning. If the front driveshafts are not turning when the transfer case is in four-wheel drive, the problem is inside the differential (broken side gears) or the transfer case (broken chain, shift linkage, etc.). If the driveshaft between the transfer case and differential is turning, the problem is in the differential. If the driveshaft is not turning, the vehicle is going to need repairs to the transfer case.
Hope this helps.

There may be vacuum lines running down to the hubs. If so check them for vacuum when engaging and dis-engaging 4 wheel. If you can't feel any vacuum, trace lines, and check for leaks.

Sounds like one of your hubs is sticking and not engaging. You can check them by engaging the hubs and the transfer case and jacking up each front wheel independently if it doesn't spin your good, if it does you found your problem. As far as fixing a hub goes, it may be just dry and need some grease, or there are some very good aftermarket hubs available from Warn.
Hope this helps

Try removing the front drive shaft from the transfer case to the front differential. With the vehicle on 4 jack stands and the front drive shaft removed attempt to place the vehicle into 4 WD. You may have to start the engine if the vehicle uses a vacuum actuator module to engage the front wheels. When you get an indication that the vehicle is in 4WD try turning the front wheels by hand and see it you hear the same grinding noise .... engage the hubs and try again. Usually if the transfer case is ok then the problem is in the front differential or one of the many universal joints that connect all of the elements together. Check the U joints on the front drive shaft for play as well as the U joints on each of the front wheels.

i'm assuming you have a 4 wheel drive. If that is the case then yes cause when your in two wheel drive mode the front is in neutral

I take it the transfer case shifter feels okay (shifts through all four gear positions), so try jack up one side of front end, rotate wheel while looking in knuckle to see if drive axle turns, if it does, then hubs engaged. Now if someone holds the front d shaft, and wheel won't turn, then both hubs are engaged. Next put t case in 4xhi or 4xlow, if you cant turn the wheel, then the t case is engaged, and all is normal. If wheel still turns at this point, then the t case is not engaged for some reason, but post again if it does. Hope this helps.

OK If you have manual locking hubs, one could be acting up. Jack the rear wheels off the ground and block the front wheels. Be sure you have the trucks back axel supported, not just on jacks! Then, with a person in the drivers seat, put it in gear. The 'driver' will turn the steering from lock to lock, while you check [from outside the truck, not under]] for the shaft at the front wheels that is spinning. This is the prob ! Frankster016