Are you driving that route a lot? Are you cruising over 70 mph?
Driving habits, environmental conditions, and truck weight have just as much to do with it. If you wanted maximum fuel economy then a 2WD Tradesman is what you want. V-8, 4x4, ORG, tech packages, etc., all add weight to the truck. They don't seem like much individually but it adds up and you have to decide how much comfort/convenience you want at the sacrifice of economy.
The longer the list on the window sticker the less chance you have of seeing those MPG numbers posted on it. A Laramie already has a base package with included comfort and tech options that add weight. If you've done anything else to the truck since you bought it (lift, oversize/E-rated tired, etc.) those are also economy killers.
There's plenty of 3.21 vs 3.92 threads around and if this was 4-speed I'd say go for the swap. Since we're talking 8-speed there's very little to gain, especially for what it's going to cost. It would take a long time for you to come out ahead at the pump in that deal.
I read of guys with 3.92s getting better mileage than me (and I have 3.21s). Too many variables for me to feel like I got the short end of the stick.
I didn't buy an all-motor V-8 for the fuel economy. It's not like I don't care (10 MPG would make me a little peeved) but I certainly didn't expect the window sticker (17 MPG overall on mine). My expectation was 15 mpg (which is what I got in my last truck...F-250 diesel) and that's right about where I'm at. Good enough for me since I've got a long list on my window sticker.