hello, and welcome. with vehicles so large, your best bet is driving techniques. aero mods could help some, but adjusting the way you drive will yield the cheapest, easiest, and most noticeable gains. a scangauge will help...http://scangauge.com/
before someone throws rocks, i'll ask politely...do you really need 2 gas guzzling vehicles? many here own trucks and suvs that are functional. others still, have smaller and more efficient daily drivers in addition to their V8/V6 truck or sports car.
maybe some truck/suv owners could weigh in on their mods be it lowering, grill block, tire inflation, etc. Jay2therescue lives in VA and owns a truck w/ mods. and he's a "mod"erator here at the site! he's got some impressive mods, and i'm sure he won't mind me stealing his thunder so here you go...http://www.gassavers.org/garage/view/1868
Welcome, Greg. Jay and I both have GM full size V8 pickups that we try to drive efficiently, and there's quite a few other members with medium and full size pickups/SUVs. The driving techniques are mostly the same.
The first and most important thing to do is to track your fuel economy accurately. Enter your vehicle into the GasSavers.org Garage and start a gaslog. Every time you get gas, record the miles you went on the last tank and how much you put in. Don't top off, stop at the first click; and if it's not inconvenient, get gas on the same side of the same pump each time (to get accurate first-click shutoff).
These vehicles often have tires that can accept higher pressure than most, and some people (me!) can use very high pressure without any compromise in ride, handling, or wear. The increased tire pressure, if it doesn't compromise any of those things, can help fuel economy, is totally free, very easy to do, and doesn't require you to adjust your driving.
Automatic transmissions often don't operate the most efficiently if you accelerate too softly. Once you have a good baseline in your gaslog, experiment with different acceleration styles, each for a few tanks, to see how they affect your fuel economy.
Of course, if you want to, you can always gain fuel economy by slowing your cruising speed...but you surely knew that already.
Grgeg - Welcome to the site! Bowtieguy and theholycow gave you some very useful information to start you on your journey. There is a plethora of information on this site and very knowledgeable people willing to help. So jump in and enjoy! I'm running a 07 GMC Sierra with 5 spd MT and many bolt on mods. But the Scan Guage and modifying my driving techniques has really helped my MPG's. Good Luck and keep us posted on how you are doing.