2000 GMC Sonoma Push Button Four Wheel Drive won't go in 4 High?

[DE1]

I have a 200 GMC Sonoma. It has the push button four wheel drive. I can't get it to go in 4 High. I push the button and the light flashes four times and then it stays in two High. I can get it go into four low fine but not four High. Anybody have suggestions as to what might be causing this problem and how to fix it?

 

[Price]

There is a little gadget in the transfer case with wires going to it. It actually uses a small heating element to activate the selected mode the transfer case will run in. Sometimes it is the wires going to the unit that are compromised. Sometimes it is the unit itself. Its not the easiest thing to get to, as it is on top of the case, hope this helps.