Re: 4x4 won't go into hi
I am by no means an expert on anything but I have a few suggestions that will not cost any money to do.
I would suggest looking at the electrical connections on your transfer case. I found that on my car the salt and corrosion has fouled up connections on my starter and AC pressure switch. It takes little time to disconnect and check to see if the copper has turned green. If so, spray some electrical contact cleaner and try to clean the connections as best as you can. This may solve the problem.
Another suggestion I had, was to check the amount and colour of your transfer case fluid. All it takes usually is an Allen Key to open the top plug "check plug" of the transfer case. The fluid should hopefully be pinky colour, usually ATF however, I have seen some of a green colour in GMs. The level should be about 1/4 to 1/2 inch below the hole, however truly full is when it dribbles out of the plug opening when you remove the plug. If the fluid is worn or low, this may be another reason to your lack of 4X4 HI. Being as the truck is an 01, I doubt this is the case (without knowing the mileage) but its worth a look. It doesn't cost anything, and you should be able to do it in your driveway/garage.