Hello Andy
I do not blame you for not trying to reverse at 50mph rear wheel steering is exciting ( read terrifying) I think your problem must lie with prop shaft, maybe alignment and/or balance it is really the only thing that is "new" that is likely to cause the problem.
Always difficult if you are remote, but the propshaft looks pretty close to the cross bracing in the transmission tunnel, maybe an just an illusion.
Peter Gamble may send you a new prop to try, it seems to be the most logical.
Glad you got home OK , good to see you both, a great weekend I thought all round, great cars, people, venues and weather
Kind regards
Trevor