Scary steering shaft/coupler failure!

I don’t have my fsm available but there is an adjustment procedure in it to ensure the coupler shoes are properly engaged depth wise in the coupler housing. I don’t remember specifics. If they’re engaged too far it can cause binding as they will bottom out. If they’re not engaged far enough I could see them coming out the open end of the coupler. The “depth” is set by sliding the whole column in or out on the three mounts under the dash. I know you said you hadn’t messed with the column but you did change the gear box etc. That could have changed the engagement depth due to differences in accumulated tolerances in the components. Other thing is the input shaft on the steering box could not be properly preloaded and the gear is walking in and out of the steering box. New or rebuilt parts are not necessarily good all the time. But Firm Feel has a pretty good reputation.