Being unsure of the Getrag, I'm glad I have the NV5600 in mine as it's a proven unit. I just wish it had another gear or two to slow the engine down at highway speeds. As for the AA axles, are they weaker in some way or just unknown? My window sticker reads that with the 6 speed, I got an 11. 5" rear axle. Seems pretty large but don't know what the previous Danas had in them.
On my 89 Ford diesel, it's 5 speed had a different 1st gear than comparable gas 5 speeds. I think mine was in the 4:1 range compared to a gassers 6:1. It was the ZF trans. Point is, the ratios don't have to be the same on gas vs. diesel.
Jim