Clay,
I broke a left front axle last year and had a hard time find finding parts for it. The only available axles were the LR genuine ones. GBR doesn't make one as they are trying to design a HD CV joint to go with it. Last week I broke a right front CV joint and to replace it you need to replace the entire axle as buying the new part comes as 1 piece.
My recommendations are to buy new HD axles from Great Basin Rovers and keep the left over good parts as spares.
The main problem with the D2 is that they are too new so there aren't many parts available for them. They are also a bit more complex in the front than the D1.
As for how you broke it...many factors play into that including tire size, obstacle you were attempting, driving style, and wear and tear on the axle. Many people break axles in their driveways...the torque stress from wheeling finally broke the axle when they placed torque on it in the driveway.
As for me, i have 33" BFG MTRs with all stock axles. Works well for me except in the front.
Jamil