The four wheel drive quit working while driving in 4 wheel high on a back road. The truck is a 1991 Chevy Silverado CK 2500. The front axle drive light on the floor shifter does not light up when shifting into 4 high or low now.

I am not familiar with the model. I wonder does it have differential locks? If it does, that could be the trouble.

In order for an axle to lock there must be some fairly serious trouble with it - something broken inside.

It is impossible to drive a true 4 wheel drive vehicle on a hard surface because the front wheels (in a forward motion) always travel a greater distance than the rear wheels which results in wind-up between the axles.
When such things as crude old army vehicles had to take to the road they would be forced to stop periodically and one wheel would have to be jacked off the ground to relieve the wind-up before they could continue.

Regular road-going 4x4s have a unidirectional clutch fitted in the front drive to allow the front wheels to overrun the rears without wind-up. When diff-locks are engaged a road-going 4x4 becomes a true 4x4 and axle wind-up becomes likely.

From your description I speculate it did not get out of 4-wheel high but instead engaged the diff-locks and it became a true 4x4 and you experienced axle wind-up, something travelling in reverse will relieve.
I further speculate the fault has since corrected itself. It remains to be seen whether it was a one-off glitch...

The best way is to take your time...,Set Emerg brake,start vehicle and put into neutral..Engage 4 wheel drive,look at your dash gauge area for 4 wheel drive light to come on..Put truck in reverse and release park brake..,Back up 10ft.slowly..Then put it in drive and pull up 10 ft.slowly then you should be in four wheel drive..

Is it both front wheels or just one? This will help work out where the problem is. If it is just one wheel, the problem is between the wheel and the front Differential (Diff).
(The fat bit in the middle of the front axle).
If it is both wheels, the problem is the Diff itself OR further back up the system. Parts like the drive shafts, linkages and the centre transfer box. Go see a good 4 wheel drive specialist. That'll be the easiest way to sort it out.

scrap it

Actuator on front axle not working or unplugged ,,

push button type floor shift type high range2wd does transfercase motor make noise did you burn transfercase clutches out? front not engaging but low range for rear help us help u

you need to stop both front wheels from turning to see if 4 wd is working
just stopping one side means nothing as the diff action is operating and making the opposite wheel go faster

take the four wheel drive actuator out of the front differential let it hang while still plugged in and have somebody watch and see if it retracts when the 4 whell drive is switched from 2 to 4 whell drive. If not then accuator is probably bad.

Sounds like the chain in the transfer case has broke or the linkage is out of adjustment

Your problem is in the front axle.The actuator is no longer working it also is a common problem....

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