Okay, well a Terra Grappler isn't near as heavy as an MT. So as I said the 305/55 shouldn't hurt your MPG too bad from stock. It's just the extra weight and rolling resistance that you'd have to consider. I'd guess you're looking at a 1 MPG drop max. But yes, you will need 1/4" spacers in order to make them work. The suspension set up on the 4th gens is much different than your 03 so you can't really compare it to that.
With the 285/65's you're probably looking at around a 2 MPG drop max. Those you will be able to fit on the stock wheels with no spacer.
As far as the recalibration goes I believe the dealership can do it. I've heard they have limits on how big of a tire they can calibrate it to though. Can't remember exactly what they are off the top of my head.... Hopefully someone will chime in with that info for you. Otherwise you can probably find it using the search engine. Another option would be the Hypertech Speedometer Calibrator. I think they're around $250 (shot in the dark)