Ive used the kent moore tool on my 04 tahoe, same set up as your avalanche. Just use the tool to press upagainst the dimple on the torsion key and unscrew the bolt in the crossmember, back out the threaded rod in the clamp to unload the torsion key and slide the bar out from the control arm.
Looks like this one http://www.amazon.com/dp/B0002SRGYI/?tag=atomicindus08-20
Disconnect the shock and upper ball joint and you can droop the lower a arm more than enough to release all preload on the spring without the clamp tool.
Upper ball joints on those IFS GMs are low service life anyway. Might as well just replace them.