96 500 RMK- Driveshaft Help

Need Help- Currently have a 96 500 RMK, that I put a 1.75 x 136 under a few years back. In order to get the clearance I needed I also had to change the drivers from the 9 tooth 7" to the 8 tooth 6". This had really worked great for about 5-6 years until last year when I broke the driveshaft in half. I replaced the driveshaft ( with a used shaft) and was able to get about 300 miles on it when it failed again last week. My question to all is- By putting this aggressive of track on this machine am I putting to much torque on the driveshaft which is causing it to fail? I did not change the gearing from what came on it stock, but I do not know what is causing it to fail.
Any help would be appreciated, since the kids love this sled to ride.

Thanks
Grandpa

Does anyone know how I can get hold of Indy Dan ?

Thanks

Grandpa

After breaking the orginal I replaced all bearings etc ad set the track tension to 3/8"-1/2" per the owners manual. I too am wondering if by chance the "old" XLT shaft I used ( I had to press on my drivers) was bad to begin with? Just didn't know if any of you had run across this same thing or if something stood out to you guys of what I was doing wrong or if using a 1.75 x 136 was casing too much torque on the shaft. I haven't torn it down yet, but also curious if I had a bearing seize up? Would a seized bearing cause the shaft to snap in half?
Grandpa

I would think a would hear the bearing first as well, but since I wasn't riding it at the time I can't same for sure- My girls are pretty mellow riders, and we were on the fresh groomed trail just going to Big Springs in Island Park when it hapened ( 3 miles in to our 4 day trip- bummer) As for the shaft everything measured out the same and went together like I imagined. It did have set screws to "lock" the bearing in place on the speedo side.
Thanks for your help, I think I will get another used shaft and give it another go, at 20 yrs old the sled means more to me and the kids then just parting it out. I guess it figured I needed more wrenching practice
Thanks again
Grandpa