For exact axle availability download the Ford Super Duty Order Guide and Tow Ratings. As I read it the 3.55 axle is available only in an F-250 7.3 10 speed. All other gas engine combinations are 3.73/4.30. In an F-350 the 6.2 is available with 6 or 10 speed, but in the F-250, only the 6 speed is offered. Also, noteworthy is that the all the 6 speed combinations are the TorqueShift-G transmission which is a beefed up F-150 10 speed; not the 6R140 used behind the larger engines in earlier years. If it were my money I would definitely get the 7.3 (only comes with a 10 speed) with its great, flat, torque curve and a 3.73 axle (tow rating of 16,000 lb.) in an F-350 (for a VIN sticker and springs rated for the higher weight.)