Diagnosing Suspension Sag - Help Please

[KMK454]

I noticed that my truck has developed a sag towards the driver's side. It seems to be even across the front left and rear left tire. I'd like to diagnose it this weekend and get working on a solution.

Aside from checking air pressures, what are some good places to look at for diagnosing the problem? The rear leaf spring does appear to be straightening out a bit. The shocks are ancient and the springs are original to the vehicle. All bushings are also at least 15 years old.

My gut reaction is to just buy new springs, shocks, and get a "rebuild kit" for all of the rubber bits... any ideas? (and those replacement parts need to leave it at a stock ride height, sorry ) Are the LMC springs and KYB shocks worthwhile?

Anbody use this kit for the rubber pieces?
http://www.performancesuspension.com...oducts_id=5829

Thanks!