New R&P, blows up..why?

[TDW]

I have a 62 year "P" series third member. It has had a 4.56 in it for years. I pulled the pig and took it to an experienced rear end guy, who has done many before. He does it for a living. I had a Richmond 4.11 installed. All new bearings. Reinstalled the pig, filled with Amsoil Synthetic lube , added the posi lube. Took the car for 4 10 mile trips, letting it cool for 30 mins between drives. 1,2,3, sounded good. Had a howl in 4th at 2500-3500 rpms. It wasn't consistent. Seemed to come and go. I read all I could find about gear whine , howl, and noise. This symptom wasn't found in the reading. I did a small burnout, took it for another ride. Ran 1st gear out to 6000, hit 2nd pretty hard. It blew. The bearing retainers are both broken. It destroyed the ring and pinion, the bearings are floating in the carrier. What happened? It looks like the entire carrier moved where the caps are. Thanks.
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Tony