With dual wheels I suppose nothing wrong with the 5/10 lug stuff in a 1 ton and if you like 19.5 tires/wheels as your only available size.
I did a similar swap in my 1 ton back in 04 with a Dana rear axle.
Here is some service info on the 12" Rockewll axle.
http://vintage.mitchell1.com/PClubData/chassis/ent82/V2D829043.pdfAxle Id info here
http://truckpartsetc.com/sales/PDFs/Leland/Leland_AxleID_006-011.pdfI'm going to guess it is a Rockwell model c100 or C102.
You might find some parts but they're certianly not as plentiful as the Dana
http://www.canadawideparts.com/downloads/catalogs/meritor_outOfProductionSingleReductionAxles.pdf page 2
If this is the correct model it looks like gear ratios were at one time available for as fast as 3.70 and plenty slower. If not already in the axle you get finding a desirable ratio might be challenging or expensive.
Looking in the parts catalogs it seems the 10 lug Rockwell and Dana axles under the P30 all use the same wheel hubs and pretty safe to assume brakes.
Were I in your shoes that Rockwell better have the
exact gears I want. Otherwise even if it were "free" I'd instead go pay money for a Dana I can easily get gears and parts for.
