steering rack bushings

Slop in your steering could be the shaft rag joint, rack bushings, worn inner or outer tie rod ends, or worn ball joint(s).

Have a buddy sit in the car and rock the steering wheel back and forth. Look at all the steering components. See what moves and what doesn't. If some parts of the steering system are moving, and some aren't, then whatever is in between is bad (like the rag joint for example).

As for the rack bushings, yes polyurethane is good. You can get Delrin or solid aluminum but they cost more and don't insulate as much. If you know that you're going to upgrade to 5 lug at some point, you may want to invest in offset rack bushings. You can rotate them to move the rack around and help eliminate bump steer. Same goes for the outer tie rod ends. If they both need replaced, it would be wise to invest in a bump steer kit, since we all know that we never stop upgrading our Mustangs.