2001 XLT 4WD disengage problem

My 4.6l xlt has almost 140k miles and just recently after engaging 4WD i could not get it to disengage when I switched it back to 2WD with the dash switch. Eventually it did disengage with a bit of a clunk. It has happened twice now and both times were on really hot days (about 100F). I normally only use 4WD in the snow in winter so i do not know if the temperature is significant, but it did not have this problem last winter.
Any ideas what the problem may be and what is the likely fix.
Thanks

service the transfer case it sounds like ur gears are gummed up also u should at least once a month use your 4 wheel drive for 15 min ato keep everything lubed and cleaned

Do you have auto hubs or man?

Auto hubs, with a dash mounted switch to change from 2WD to 4WD Hi and 4WD Lo
the switch between 2WD and 4WD hi can be made on the fly

It may have something to do with the hubs not fully engaging or disengaging. My truck is a 94 so its older then yours and i have push butten with auto hubs. And i notice that when i would put it in 4 wheel and take off there would be a cluck then it would engaging. Then it would be a pain to get it out off 4 wheel. So i changed my hubs to manuel and have not had the probem since. Or maybe there is a vacuum line that is dry rotted or nocked loose from your transfer case. I dont no if any of this will help u but good luck.

These newer rigs don't use vacuum. They use an electric motor to shift the case. They can go bad. The module knows what position to shift based on the different resistances in the switch positions; don't use the switch and it gets dirty contacts and it doesn't know what to do with its numbers.
What was said about auto hubs is true. that can also be avoided if you use it once a month to keep it lubed up.
Manual hubs have less moving parts there fore they are not as succeptible to this. Auto hubs work fine if you use it often enough to keep it lubed up inside.

I am now feeling pretty stupid about this!
The problem had nothing to do with a pending drive failure, but everything to do with the new tires I had replaced on the front whilst keeping the old (worn) ones on the rear. The difference in circumference was sufficient to create this problem. I have since replaced the rear tires too and the problem has gone away.
Another lesson learnt!