sdhawk

I have a 1997 Ford F-150 that I really like and want to keep but it has a 308 gear rear end on it and that makes it lousy for towing my boat. It really strains to pull it. Hard to get to speed and hills slow it down. It has the 4.6 V8 engine. I am thinking about having the gears changed out to 355 gears by a Ford dealer. The cost will be about $1100.00 total. He said that should solve my towing problem but will probably lower my gas mileage 3-4 MPG. I now get about 16 mpg around home. The dealer said it may be worse because the transmissions are matched to the rear end that is on the truck. I knew there were different gear ratios for the rear ends on vehicles but I figured they had the same transmission. I was wanting your opinions on this from guys who know what they are talking about. I'm no mechanic. I have been thinking about just getting another vehicle and might but I would like to keep this truck if this is feasable. Any other ideas? Is this something that is going to give me more problems? Someone mentioned a gear vendors underdrive but I really don't know what they are talking about.
Thanks,
sdhawk