I have one thought on what it could be. Check and see if the two wheel drive trucks have a rubber donought on top of the coil springs. On the four wheel drive's they have this piece of rubber between the spring and the upper spring tower,your's could be wore out and causing the sqeak. I would think that the noise would be constant when the springs react not just when the wheels are turned. JMHO.
I hope this will help and good luck.
My $ . 02