how often do rear coil springs go bad?

[theastronaut]

Do the bounce test right after driving it. I've seen shocks go "limp" if they've sat for a bit and might need to be cycled a few times before they dampen like they should.

The thrust angle would probably be the panhard rod not being the exact length it needs to be for your exact ride height. They make and adjustable bar to help with that.

Before buying new shocks, add a few hundred pounds of weight in the bed to see if the bouncing gets worse. If the shocks are bad they'll have a harder time controling the body movement and the bounce will be worse. If it rides better then the shock valving is too stiff for the amount of weight on the truck when unloaded and the shock isn't allowing the suspension to absorb bumps like it should.