In 2000 GM started using torsion bars in the front of all their full size SUV's, 2WD and 4WD. They share the front suspension with the 4WD Silverado & Sierra platform. They can be lowered with keys, control arms or knuckles. If using knuckles you need to use at least an 17" wheel to provide clearance for the ball joint nuts with the new knuckles (Note, some factory 17" wheels are too thick and do not provide clearance) Knuckles are by far the most popular way to lower the front.
In the rear these vehicles now come with a coil suspension. The coils are the same design across all the different vehicles but there are some important differences to keep in mind that will affect the amount of drop that you get. Such factors are;
Weight (Avalanches are lighter in the rear than the other SUV'S)
Shock style (The Premium heavy duty shocks actually provide about 1 inch of rear lift)
Rear Air Ride (These vehicles need a new sensor link or else the compressor will keep trying to put it back to the OE ride height)
So be sure to carefully check the application chart when ordering. BellTech has made it easy to figure out by including an adjustment spacer on their Pro Series coils.