Too much torque

You may also want to look into your pinion angle. I've run 500pound feet of torque thru the small joint/yoke before with no issues. Right now there's a larger joint 4sp/dana combo in a heavier car making close 600tq. So you might want to look into driveline angles and how the rear moves when you drop the clutch or shift hard. If the rear rotates too far "up" the joint binds and things break.