For Ram 1500s, its very easy. The axles were all the same. All Ram 1500s came with a Dana 44 front axle and a Chrysler 9.25 rear axle. The Off Road editions came with a reinforced (some call it "trussed") pumpkin/differential on the front Dana 44, but it is still the same axle

For Ram 2500s/3500s, it gets a little more complicated.

For the front axle, the transmission/engine doesn't matter...
1994-1995 LD 2500: Dana 44
1994-2002 HD 2500: Dana 60
1994-2002 3500: Dana 60

For the rear axles, it depends on what engine/transmission they came with from the factory...
1994-1995 LD 2500: Chrysler 9.25
1994-2002 HD 2500 5.9L gas V8: Dana 60
1994-2002 HD 2500 V10/diesel auto: Dana 70
1994-2002 HD 2500 V10/diesel manual: Dana 80
1994-2002 3500: Dana 80


Source: http://dodgeram.org/tech/specs/Ramspecs.htm

 

Dana 70HD, and Dana 80 Service Manual. (PDF file.)

Additional Dana Axle information and Manuals.