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I have a 2003 Toyota 4Runner of which was pulling to the right at varying speeds which made me believe it was a caliper sticking. After total examination, the rear passenger caliper was completly frozen. The fronts had varying degrees of wear on the pads and rotors. I have replaced all four calipers, rotors, and pads with Raybestos parts, as well as insuring that the floating pins were greased with brake grease. After two weeks, the rear passenger brake is sticking at different times while driving at differnet speeds. Can someone tell me if another problem exists in the total brake system that could be causing this issue??

Why did you have to replace the calipers? Unless they are pitted, a little cleaning and new seals should have done for job for far less.

Did you flush the brake system? I am unsure if the 4 runner has a, dang, forgot what it's called, but it's the thing that adjust the rear brake pressure based on how much weight you have in the back. Worth a look.

All four of the rotors had uneven wear differing from inside to outside of the rotor. They were all worn smooth on the faces front and back, but the thickness varied from the inside to the outside of the rotor. I like your idea of changing the brake fluid in the entire system and I think I will add to that changing of the rubber hoses on all four wheels.
If someone else has had this experience with any of thier Toyota 4Runner's, Tundra's or Sequoia's, I would love to hear from you.
I have also seen some blogs on the internet of people that have had the same work done and paid $1100 to $1600 at the dealership, I purchased high performance parts from a local parts house that does not deal in the junk that you buy at the chain parts houses and I only paid about $350 for parts and about 2 hours of labor at home.

That would be the LSPV - Load Sensing Pressure Valve

I've had the same problem on my 2003 4Runner but on the right (passenger side) front. Had to have the caliper replaced. Out of warrenty of course. Have had this problem as well as an A/C that does not work and a leak in the radiator or hose that's been there since new but the dealer can't find. Still there after 77K miles. I'm not a happy Toyota owner. This was my first one and will likely be my last.