What year is your Bronco TX? Power or manual steering?
Do the test that was described above, but make sure that the tires are on the ground and the full weight of the front on them, You don't do this test up on jackstands, as it won't give the same results.
You can test some things up off the ground, but for things like trackbar mounts, frame cracks and others, you want all the torque of the steering system pushing against the weight of the vehicle.
One other thing that can cause a lot of wandering (besides a loose or worn trackbar) is worn out tires. Jeff mentioned loose tires and wheels, but worn out off-road tires can surely make for a miserable drive down the road.
Even incorrect air pressure can add to the mix, but wear is a common issue.
I'm betting you've got a little bit of "all of the above" messing with you.
Good luck.
Paul