On a stock truck you can get 34 inch tires to fit as long as they aren't too wide. For 35 inch tires you'll need a 2 inch or greater lift kit. 12.5" wide or less will work with stock wheel offsets.

You might want to change the axle ratio as it's going to feel a little down on power. Also, a tuner where you can adjust the speedometer so it's not off.

 

What tire size is on it now, and what axle ratio? You'll see a code on the driver's side door frame, white decal. Post that code here. I'd probably go with a 3.73 if you have a 3.31 ratio now, 4.11 if you have a 3.55 or 3.73.

 

Depending on the PCM code, the factory speed limiter is between 99 and 105 mph.

You have 30.5 inch factory tires with a 3.31 electronic limited slip axle. Going to 305/70/18 tires (34.8 inches) you'd need a 3.73 to be closest to stock engine speed and speedometer reading, if you want about the same amount of power. A 4.11 gear ratio would help more since the tires will be heavier. Either way it won't be cheap since its an electronic locking differential.

 

3.73 gear is going to get you there without having to buy a tuner to adjust the speedometer, at most 1-2 mph off at highway speed. And since it's a standard ratio (4.11 isn't for that setup) you can probably buy the entire axle from a salvage yard pretty cheap.

 

Its the PCM limiting it. Ford does this for a multitude of reasons. Tire speed ratings and even more important, the drive shaft is not rated for high speed and if a u-joint lets go at that speed it WILL catapult the truck, killing you.