Gee...I guess I'm lucky... I'm running about 5" of rear lift and no vibration and I have 50k on the lift...also Dennis has over 100k on his and he never mentioned trouble with vibration. Don't know how much lift he is running.
Anyway...since spring rate is the topic, I can't remember exactly, but the Sporty rear springs were 100 or 110 pounds/inch however, they are progressive springs and I never tested them past 2". Hope that helps.

Everet

WWW.UPYOURKIA.COM White 96,4x4, man.hubs,K&N filter,3row rad, twin 16" fans, 3" body lift, 4" UPYOURKIA LIFT KIT,265/75 16 TreadWrights on DC2 wheels,140 amp alt. dual batts.