Rockwell 2.5 tons have 6.72 gearing. GMC 2.5 tons have 6.17 gearing. The Rockwells are the top loaders and the GMCs have a front load 3rd member (like a Ford 9"). There is much more available for the Rockwells as far as aftermarket parts. I don't think any of them came with lockers, but a Detroit can be added. It fits in place of the spider gears and reuses the case just like a 14BFF does. A very good source for aftermarket parts is Ouverson Engineering. They make billet shafts in stock or 2" sizes, lockers, spools, hubs, (all with alloy material), and a sweet 8 lug conversion hub that allows use of an 8 x 6.5 lug pattern.
We put a Super Duty with a PSD on Rockwells and 49s. If I recall it gets about 12 MPG. Not bad for a truck that size. I think properly matched with correct tire sizes, the mileage can be decent, but overall it's just the axle and size you'd want and not worry about mileage.