Doug D said:
Diesel engines don't have a stellar reputation in the US thanks to GM's fiasco in the 80's.
And apparently you haven't being paying attention. It has been confirmed Ram is making available the 3.0L VM diesel for the '14 Ram 1500. The same engine is also destined for the '14 Jeep Grand Cherokee. The estimated fuel mileage is something like 18-19 city and 28 mpg hwy. Expect a $4500 premium for the diesel. The Cummins is nearly an $8000 premium in the HD's. Hp is rumored to be 240 hp with torque around 420 ft-lbs. The Hemi is 390 hp/401 ft-lbs.
I don't believe there is a EPA limitation, but there is a tremendous cost to certify and engine/transmission setup regardless of whether it's gas or diesel.
Your D150 is couple of things going against it for good fuel mileage. Today's trucks have 5,6 and now 8 speed transmissions and the /6 has to work harder to do the same work.
Oh, I didn't know they had some smaller diesels coming up. That's awesome.
Yeah, my '87 Ram D150 had the 3 speed automatic 904 transmission. My mechanic father said it was built to be like a useful piece of equipment and not really made for highway cruising. It was a cool truck -- short bed, ice blue, last year for the 225ci Slant 6 in North America. I bought it off the original owner in '02 for $1000. It only had 90k miles and no rust. It had no options except for the auto tranny -- had a Chrysler AM radio, no carpeting, no AC.
It had real beefy heavy-half leaf springs in the back. I hauled about a ton of gravel once, and it sure was squatted down in the back, but I got it home with no problem (just a few miles from the building materials place). I sold it 5 years later to a guy who still uses it everyday to haul scrap metal. It's all beat to hell.

I wish I still had it.