GM-GMC: Axle ratios, axle ratios, gear ratios

Question
I own a 2004 GMC 2500HD 2WD 6.6L I love my truck but have noticed my gas mileage is poor (13-15mpg Hwy) compared to friends of mine that own same year model truck as I own that are getting 20+ mpg. I looked into this and found that a main cause in difference is the gear ratios. My question is would it be a wise and smart choice to change out the gears in my truck in order to achieve better mpg? I do a lot of highway traveling and occasionally haul horse trailers around any suggestions?

Answer
If you are going to do a lot of highway driving a difference of 5-7 mpg can really add up.  The only consideration really is the amount of weight you are pulling.  The higher gear ratio will definately help the bottom end when pulling heavy loads.  There are two ways you can go about changing the gear ratio.  The first being simply change the gear set, gears can be expensive and definately need to be installed by a competent technician.  The second is to try to find a good used rear end (complete) with the gear ratio you want and swap the entire assembly.  I would say you should work out the fuel savings vs. the cost and see if it makes sense to do the swap.  
Hope this helps