I as well have been thinking of doing a 6" and I have a 04 YD.
I installed NEW air shocks and compressor hints why I want to retain it lol. I currently have 33's and no running boards or stick-on panels with a z71 rear bumper. I added the tires the others stuff was done prior to purchase. Looks good most people dont believe its a Denali. LOL!!!!
fabtech, superlift, procomp have 6" lifts that utilize the factory rear shocks and skyjacker has a 3"... anyway you look at it everybody says anything over 4" you have to change the front drive shaft
SUPERLIFT
"does not require cutting the front diff mount" keeps factory steering geometry as well as cv angles. But they say your need to replace the front driveshaft....
"Kit includes front dual shock mounting brackets and 6 shocks with yellow boots (2000 - 06 Tahoe / Yukon have 4 shocks and boots and utilizes stock rear shocks)."
ProComp
doesn't really give any info on this
FABTECH
this kit requires trimming of the front diff (you have to trim a few boss's off and CUT THE FRONT MOUNT OFF and install a new bolt-on mount). BUTTTTTTT by doing this allows the use of factory front drive shaft in 4WD models idk about the damn Denali's though.
One problem with this.... I have installed this kit in the past and it only drops the Front Diff 3" this is that kit you have the steep angles on the cv axles and if you do off-roading it blows the cvs apart so your going to want to buy some spare cv axles lol!!!!
We nicknamed this the pretty boy lift lol!!! It's the cheapest out of the big name brands but its really only made for lifting for looks and city driving!!!
Me, personally I'm doing SuperLift with no expenses spared I'm doing it right. New ball joints, inner and outer tie-rods, pitman and idle arms, and installing the Superlift front drive shaft it allows more flex with the suspension and supposed to delete the vibration on highway speeds. Also I'm installing the steering stabilizer to help on the excessive wear to the components....
I hope this helps any lol