[Too Many Projects]

The consensus was the drive shaft was not able to handle the rotational rpm and was bowing in the center, like a jump rope and the vibration caused the break. I have a solid motor mount on the left side and a urethane on the right. The trans mount is rubber, to allow for any slight twisting that could occur. Twist isn't the issue here, as the breaks we've experienced are happening behind the trans mount. As a result of the replies here and talking with driveshaft builders, I now have a 3.5" heavy wall aluminum driveshaft that is balanced to 6,500 rpm with solid 1350 u-joints and yokes.
Your driveshaft is a little shorter and you should be able to get a heavier wall steel shaft that still meets the critical rpm rating. Hopefully that is allowed in pure stock. If not, have the stock shaft checked for high speed balance.