i still can't see it being viable. historically, going from a small block gas to a big block gas was several hundred dollars to a thousand more in cost. going to a diesel was several thousand more. then the possible addition of a heavier transmission can add a couple thousand more... i can't see any of the manufacturers making a 1/2 ton upgrade to diesel costing less than 5k-7k, and that's a huge hurdle to overcome when trying to justify a diesel.
i've yet to drive one, or even know anyone with one, but i've heard of 20+ in a 4x4 ecoboost f150. the ecoboost is about an $1100 option over a v8 (i looked it up), so i'd guess a diesel in a 1/2 ton will still be about $5k over that. let's say the fictitious 1/2 ton diesel can get 27mpg in the same 4x4 trim. at $3.50 for gas and $3.75 for diesel (pretty close to what it is here right now, and if anything a little low on the diesel price) you've got to go about 140k miles before you break even, and that's assuming fuel prices don't change, and you can get 27mpg. i didn't factor in the maintenance cost, but yeah, there's going to be a premium for oil and fuel filter changes in a diesel. probably 150k miles is closer to a break even point really, and that's making big assumptions.