Any pics? Was it the spindle itself that failed or the bearings, hub? Not sure what you mean by it broke clean in half. Glad no one got hurt, I ask so I won't hurt anyone.
I had a trailer spindle broke clean in half on a bumper that was mounted with 10 grade 8 5/8 bolts (6 in the stock locations and 2 on each side going to the frame), the open end was secured with a grade 8 5/8 bolt through some 1/2 inch plate. It was snubbed up tightly against the rubber stops on the tailgate. Still broke.
Put a video camera facing out your back window pointed at your current configuration, drive down ............say Bangerter Hwy and just see how radically that tire is bouncing. It doesn't take an off road situation to cause cycling failures.
Glad it was bolted on the other end or that young girl in that little car following me would of probably at the least would of had the life scared out of her.
That spindle btw was welded on by not only a professionally certified welder but a NASCAR certified welder all the same so I know it was done right. That's the only part of the build I personally didn't want to screw with.