... I saw in another post that you have 3.31 gears. Upsizing your tires like that would have an effect on your rearend gears as if you changed them from 3.31 to like ~3.17...
As far as gearing goes, if you have a 3.55 rearend and 275/55 R 20 tires (32 x 10.83 x 20), changing to 305/55 R 20's (33 x 12.01 x 20's) would be the same as if you kept your current wheels and tires but changed your rearend gears to 3.41's.
Also, just only doing a level on the truck with these tires being wider could have them rubbing on the crash bars, but I am by no means an expert on this subject. You may want to do some research on f150forum.com where there are a lot of posts on tire sizing and lifts and levels.
I do like your choice in Ridge Grapplers, and I may go that route when I get around to it. I was running LT Duratracs on my 2010 for many years and loved how those tires handled on-road and off-road.
Here is a good reference site you may want to check out -->
DML Tire and Wheel Calculator