Ford rates the class II (std) Sport Trac for 3500 lbs of TT - It also appears that with the addition of a class III hitch the tow rating magicaly goes up to betwen 5000 and 8000 lbs - the payload stays the same - only the CGWR and tow rating change the engine/trans/rear ends also stay the same. Unless there is something Ford isnt telling us about - you should be able to change the hitch and gain 2000 lbs of towing. Not to mention that the class III hitch will accept the equipment for that 2 5/16 ball.
Can't find much about your Shasta.
07 Chevy 2500HD 6.0L ,6 speed auto, 4x4, Blue Granite Metalic
05 Arctic Fox 22H