If the clicking sound only happens when you rock the truck while parked, and you never notice a speed sensitive rhythmic clicking while driving, then my money would be on shocks. You can try spraying some silicone lubricant around the bushings and see if the sound goes away.
So many possibilities. Every suggestion here will be speculation. That said, some of the things I've seen that have caused a clicking have been:
Rock wedged in tire tread
Rock wedged in brake backing plate.
Missing parking brake shoe retention pin.
Misadjusted or dirty/rusted parking brake...