power steering

Check your power steering fluid, power steering hoses and steering box for leaks.
Find the leak and repair it...It is a sealed system, if you are adding large amounts of power steering fluid, it is leaking out somewhere. Hoses, reservoir, steering box, power steering pump are places to check. You should only have to add minimal fluid to the system for the entire year.
The immediate squeal you hear is most probably the unit is out of fluid or has leaked onto the serpentine belt and causing it to slip.
Fix the leak and check your serpentine belt for wear.

http://www.fixya.com/cars/t2068287-steering_issues

Sounds like your denali is equipped with hydroboost power brakes here's a web site with info on troubleshooting http://www.babcox.com/editorial/bf/bf030542.htm Hydroboost power brakes dosn't use vacuum to boost the brakes its hooked up to your power steering.

i have the same problem and i just had a power steeing box and the serpentine belt changed.
i just read about a steeing senor wire under the dash on the steering shaft that can be disconnected solving the problem. i will inform my mechanic and see what he says. i suggest you contact a mechanic also. hope this helps. i have a 2003 yukon.

That sounds very much like a problem with the power steering system. I could be an issues with the power steering pump, it could be low fluid, or it could be a leak somewhere in the system.

I'd take it to a professional to have inspected and diagnosed.