You're by far not the only one having final drives going out. We had one going out on a 2000 model STS, too. Other people have reported the same befor on this board. It didn't used to be that way before the 50 series. Would be interesting to know if JD has made any changes for the 02 or 03 models to make those last longer.
We had no choice, but have the dealer come out to the field, take it off the combine and fix it. Since the introduction of the STS, there is no extended warranty on final drives anymore either. So we had to pay for it (over three grand).
Why should a bin extension make a difference now, when all the 9600 and 9610 handled it fineIJ Maybe the set back front axle on the 50 series is more reason.