It may be due to larger softer tread blocks that tend to squirm a little when braking. If you are lucky!
It's also possible that the 33's may be overpowering your brakes. Larger tires create a leverage advantage that works against your brakes. You may be encountering a situation where the larger tires are now at or near the maximum design limits of your braking system.
You shoule probably need to not just replace the brakes, but do an upgrade. Replace the rotors with slotted rotors, and switch to a good quality carbon metalic or ceramic brake pads. If you find larger rotors and bigger/better calipers, that would be the way to go, but I don't know who makes a kit for the Sport Trac/ Also if you find larger rotors and calipers, you will likely have to go to a 17" or 18" wheel to fit it all in there.
...Rich
