My wife drove our 4x4 truck in 4wd for several hundred miles through a winter storm recently, at highway speed (she was going hunting in Montana, we live in Minnesota). She heard a “clunk” or some similar sound, and it was difficult to get out of 4wd (as evidenced by the indicator light). It has taken awhile for it to stop indicating 4wd after we shift it out for a year or so. She drove it home, in 2wd, with the manual hubs unlocked. Now, with the hubs locked, it will indicate that it is in 4wd when we shift it into 4wd, but it is definitely not. I think I can smell gear oil. It works fine in 2wd. What the heck is going on? Model-year 2000, 2.7 liter 4-cylinder, 5-speed, about 180K miles.
Jack up the front end and with the wheel hubs engaged and the PTO in neutral try to turn one of the front wheels. If turning one wheel causes the other to turn in the opposite direction get under the truck and attempt to turn the front drive shaft. If it turns shift the PTO into gear and again attempt to turn the front drive shaft. Post the results.
If this is a true 4WD system, and the vehicle was driven on dry pavement for an extended period of time it can damage the tranfer case. The transfer case is what links the rear and front differentials when the transfer case is set in 4WD. The problem with this system is it doesn’t allow the front and rear differentials to turn at different rates while cornering.
With an AWD system, instead of a transfer case there’s a center differential. This type of system allows the front and rear diiferentials to turn at different rates.
So if this is a true 4WD system, and it was driven with the transfer case in 4WD, the transfer case is probably burned up.
My guess is your transfer case is shot. Perhaps it has been bad for awhile, since the indicator light was staying on. When was the last time you had the transfer case fluid changed? What was the transfer case fluid change interval over the years and miles? A 2000 era truck with 180K miles should have had several changes of transfer case fluid (and front and rear differentials too) over the years.
If the trip in “winter storm” was on an interstate and the road was mainly just areas of wet and dry pavement it could have overheated the transfer case and caused it to bind up and eventually fail. 4WD is fine if there is actually snow cover on the road, but if the pavement is just wet it is better to go back to 2WD mode until you get into an area of snow covering the road again.
Also older type 4WD systems, which yours is if you have manual locking hubs on your front wheels, do not really like 70+ mph interstate speeds. You are best sticking to 50 to 55 mph when the 4WD is engaged.
It could be just a problem with the engagement mechanism for the 4WD, but from the info posted I think it is more likely a transfer case failure.
I’ll add to that by saying that if you feel the need to have 4WD engaged because of the snow, then driving “highway speeds” is an astonishingly stupid thing to do.