Interesting info on F250/F350 rear axles. UPDATE 01 24 14

It depends on the engine!! F250 and F350 both get the 10.5 if equipped with gas engines. F250 and F350 both get the 11.5 with diesel power.

Similar with GM trucks. Duramax (and the 8.1(?)L gas) get an AAM 11.5 axle, while other gas SRW vehicles don't - at least from ~2000 to ~2011.
Maybe the bigger axles are enough more expensive, or enough heavier as you mentioned, that it make sense to use different ones -- otherwise, no clue why they have two.