If you want an exact match you have to stick with the 01 and 02 Sport Tracs. Those 1st two years the Sport Tracs had drum brake rear axles.
In 2003, the Sport Tracs started being built with disc brake rear axles.
You should be able to use an Explorer rear axle with disc brakes with a couple of modifications. I think I've read that you would need to change the proportioning valve in the Brake Master Cylinder. And the brake lines don't connect at the same point.