Yes you would have to buy an open carrier. The powertrax you're looking at may be a full carrier replacement.
If you're serious about a sport trac I may be selling mine, set up nicely. I'm probably going to buy a newer F150 very soon. I almost traded the trac in a couple weeks ago, was driving to Indiana to do it (had been emailing their internet manager for a week), and he called me 30 minutes before I got there and said it sold

The only reason I haven't been trying to sell the trac on my own is because I don't have another vehicle to drive to work if I do sell it, the Jetta is my wife's car.