OK, you chose the 3500 configuration with a low payload, maybe the 4x4 Megacab. I don't see a configuration with a 2890 payload, but my Ram brochure shows one with 2980, and that is still a dually model (it is flagged with a '2', which means DRW). With your 750# bike and 500# of tools & cap, you definitely need much more payload with all that weight. Looks to me like you need the DRW with 4.10 gears, which gets you 4680# in a Crew Cab, DRW, 4x4 Long Box. I don't see a MegaCab configuration that gets the capacity you need.
I'm using the figures from the 2011 2500/3500 brochure - http://www.ramtrucks.com/en/pdf/2011_ram_2500_3500.pdf
There may be a payload upgrade option available that would let you use the 3.73 axle - check the options list.