I'm partial to Dodge, because we have one (2006 Dodge 2500 4x4). The Cummins engine is simply the best diesel engine available, it doesn't get any better or more powerful. My Dad is a retired Cummins mechanic, and he agrees. If you have the option to get a 2007 or later, do it. The newer models have a built in jake brake which is RAD and will save your brakes. I wish ours had one of those doo-dads.
We have over 90k miles on our truck (65 are ours, mostly towing). Whenever we've climbed steep passes, in every case we have passed Fords, diesel pusher buses, and gas-powered motorhome. We treat our engine with respect, get it maintained regularly, and it pays us back a thousand times over. Cummins. Kicks. Ass.
About the Cummins and biodiesel..another reason we leaned toward a truck/5er combination was to have the ability to make our own fuel and buy biodiesel on the road, which we've done. While it's great having the option to use biodisel, in reality it's increasingly difficult to find and when you do it's at least 2x more than #2. And if you're going to make bioD, you better know what you're doing or you risk messing up your engine pretty badly. We've seen it happen to others, thankfully not to us. Cummins will approve of the use of 5 percent bioD but nothing stronger. Anyone who does use up to 100 percent bioD will not admit that to a dealer when engine work needs to happen (rarely).
FYI you don't need to convert a Cummins diesel to run biodiesel, you only need to do that for SVO, straight veggie oil use.
About 4x4....get it. We go offroading a lot in our 24' fifth wheel, taking it to some pretty crazy places. The 4x4 has saved our butts more times than we can count, especially during the spring rains in Texas. If you plan on boondocking way out in the sticks, don't skimp and get it.
See, I told you I was biased. Good luck!