Your Schro's are going bad in the 4WD HIGH Trans Case. This sucks i know. But i had the same problem berfore.
Unit will continue to work good in low range since the stress is less then on the HIGH range. I would never use the 4WD HIGH anyway...doesnt do much on the street...just tears up the gearing and schros.
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There should be a control unit on top of the engine that controls this to keep the engine at a certain temp which runs off the engine block temp sensore. I would try changing the sensors first located on the drivers side front of cylinder head and in the goose neck on top for one of the thermostats.It wouldn't be a bade idea to change the thermostats too at the same time.
Is the Service 4WD light coming on? Do the light on the 4x4 selector switch indicate the proper selection and are on steady after a few seconds? If there is no Service 4WD light on and the selector switch light are correct, the only place for the problem to be is the Transfer Case. Has the Transfer Case fluid been changed at 50K mile intervals? This is very important.
Bit of a long shot, but check the output of the throttle position sensor with a multimeter.
Also, check that the low range switch isn't continually reporting that you are in low range. The gearbox computer adopts a VERY harsh shift pattern in low range, holding gears until approx. 3500 RPM and even then producing very hard shifts.
Try unplugging the connector that carries the low range sense signal from the transfer box under the vehicle and see if that makes a difference.
I had the same problem with my 98 sierra 4x4. Turned out that the electrical activator on the transfer case was not working. It will still go into 4 low but not 4 high. Replace the electrical switch on the transfer case and it should work again.
I had a very similar problem with my 98 gmc 2500. The 4 low would engage - though the upshifts were erratic, but no 4 high though the light sometimes came on. I changed out the electric shift solenoid or whatever it is called, on the transfer case and it worked fine after that. A fairly simple fix that can be done in 15 minutes. Hope this helps.