I cannot switch into 2wh 4wh and 4whl stuck at auto

SOURCE: My 2006 GMC Envoy Denali

MIKE,
TRY PUTTING THE TRANSMISSION INTO NEUTRAL, I HAD THE SAME EXACT PROBLEM, UNTIL I CALLED A MECHANIC FRIEND OF MINE. I COULDN'T BELIEVE HOW WELL IT WORKED.....CHECK IT OUT.

SOURCE: 2000 gmc jimmy is in 2wh drive,I switch to 4wh or

2000--gmc jimmy is in 2wh drive, I switch to 4wh or 4whlow or 4whauto and the light keeps on flashing but never engages

SOURCE: Have 2005 Yukon XL 4x4.

check all vacuum lines for any leaks?

SOURCE: My 2002 Gmc Envoy is stuck in 4wheel low. I

This is not my solution, but from mikeb33 in this forum (which apparently worked for him and others): http://forums.trailvoy.com/showthread.php?t=15584
I needed to pull a stump so I backed up my TB, hooked up a strap and put it in 4WD Low. Well it didn't take much and I was done so I switched the knob back to 2WD Hi and parked it.
Later I had to take my daughter out we jumped in and backed out of the driveway. At the end of the block, it seemed funny. I realized it was in 4WD Low still. I shifted to neutral, then park and back. I shut it off and restarted, then through all the gears. Tried every position on the selector knob.
Meanwhile my Daughter is panicking because she is going to be late so we head out anyway. As we got on the main road, I got up to 50MPH with the engine at 3,800 RPM! It was the first time I was glad I had 3.42 gears.
After I dropped her off, I pull in a parking lot and got out the owners manual. I don't think I ever read an owners manual other to see the capacities or specifications. I found that to switch back for 4WD Low, you need to be in neutral and it helps to be rolling under 2 mph. I already tried neutral, so this time I crept along and when I was well under 5 mph, and in neutral, I turned the knob. CLUNK! Back in 2WD Hi again.
Now I know what to do next time...