I currently have 14 04 and 05 Dodge trucks running the roads, 3 gas 1/2 ton 2wd gas trucks (1 being the hemi) and 11 3/4 ton (2500) 4wd diesel short box turcks;
here are a few of my thoughts:
Like most are saying - go with the diesel. The truck pictured below is a 2500 4x4 cummins diesel automatic. I have changed the exhaust and air intake and installed a volumizer chip. Currently when running empty down the hiway at 85 mph, I average between 26 and 30 mpg. No BS! Upgrading to 5" exhaust made almost as much difference as the chip on this truck when it came to mileage. The other thing that I liked about the Dodge 3/4 ton was I could put 315 75 tires under it with no lift.. factory suspension only.
Go automatic - the new automatic transmissions are so good now that this would be my recommendation. 10 years ago I would have agreed with the manual transmission but they have "jake" brakes for both styles now, the autos are almost bullet proof, with a 4x4 put it in 4-low and idle around pulling anything out of the ground. Also, check on what it costs to replace a clutch or rebuild a manual transmission on these trucks. I have had friends who have rebuilt an automatic cheaper.. They also hold their resale value better...
Go 4x4 - my Canadian roots come out on this one as it seems un-natural to go 2WD because of the what if factor - however - with the mileage I am getting on my trucks why not. As well, the extra you pay up front will be added onto the resale price when you sell it. 4x4 trucks are also easier to sell. IMO that has been my experience.
1 ton vs 3/4 ton - personal preference - I am hauling 16,000lb trailers with no problem... Not recommended but the trucks can handle it. They sit slightly higher and ride slightly rougher. They are heavier, so when driving a 2500 it seems like you have more power.. stupid comment on this board talking power to weight ratio but you get my point.
I get no better than 14 mpg with my two non-hemi gas trucks and 11 with the hemi. In my opinion, the hemi is a big disappointment. Bear in mind that most of the time I am driving a diesel and comparing it to that, not the other gas truck.
I use all my 3/4 ton trucks for heavy oil field work (when mine is not hauling my car around

and they are holding up great. I have had numerous powerstrokes (FORD) and duramaxes (GM) and those trucks did not perform in the work place 1/2 as good as the dodges. As well, I have saved almost 9,000 on each diesel over buying the other two brands - thinking of it that way on 11 diesel trucks, that is 100,000 cheaper - if you follow that way of thinking I just got almost 3 trucks for free. (compared to buying ford or chevy).
I think you should be just fine with a 3/4 ton diesel - leave the other options up to you. I think you would be disappointed in the gas trucks by any dealer.