It might be a little farther than your willing to drive but there is a shop called C&M performance in Hubertus, WI (20 minutes north of Milwaukee). The owners name is Marv and he specializes in Fords, Mustangs in particular. Fully equipped shop w/ chassis dyno. He's done work on my car at very reasonable rates and is a good guy to work with. Here's his website:
Because there is resistance applied to the roller to simulate the load you'd see on the road with weight and wind resistance. It's a more accurate measure of actual rear wheel HP than a DynoJet.