Since you probably use this on the highway I would keep the stock gearing. You have the 2v 4.6 so I would look into a set of cams, some free flowing exhaust, an SCT tuner (americantrucks.com, this will also help with shift-firmness), and if that isn't quite enough, look into a small/cheap pro-charger or a set of heads.
I know a guy with a cammed 2v 4.6 and a pro-charger that is layin down well over 400hp at the wheels. Just know that an SC will rob you of your fuel economy so bear that in mind when looking at power-adders. I think cams, exhaust, chip, and tune are your best bet. Americantrucks.com has a lot of that stuff for sale with free shipping.