From a web page about upgrading suspensions - "Be sure to use the supplied assembly lube or a lithium grease when inserting the bushings and sleeves to eliminate suspension squeaks"
And from
http://www.nyeautomotive.com/steering.htm "Vehicles used to have grease fittings simply because the original grease wasn't designed for the life of the component. All Nye's steering and suspension lubricants are. They eliminate squeaks and rattles from power steering gears, ball joints, shock absorbers, and other mechanisms. They also are engineered to resist water and salt water, tolerate high loads and vibration, and minimize friction and wear — even on rough terrain where steering and suspension components take a beating."
I've never used the Nye stuff, but have you tried anything on the bushings?