are the holes in the axle ovaled out?
I second that question.
Also, are they the correct bolts? The correct ones are shoulder bolts of a particular materials and construction but they are not always readily available and not in your average hardware store. Regular Grade 8 bolts aren't adequate in many cases.
Finally, it's important to torque the bolts with a torque wrench. That torque is what keeps them tight. Too much and they are overstretched and break. Too little, they come loose and/or break. I have seen so many guys who just run them in with an impact. If you ever work in the realm of high torque outputs and high traction (sticky tires on stocky rocks, low gearing, tons of engine torque with lots of torque multiplication... e.g. hardcore rockcrawling) you learn these things.
Also, make sure the axle flange is square with the shaft. Never seen it, but a bend housing/tube/stub axle could put the axle in a bind at the hub... and maybe break stuff there. Never saw any that caused a problem like yours but I did see some bent housings that caused extraordinary wear at the spline end of the axle shaft and in the side gear.