donor car quirk - any ideas?

[The Shadow]

Hi, I have managed to secure a 13/60 Herald as the donor for my upcoming project.
Interestingly , despite the kits no longer being local to me , I managed to find an excellent running donor just a few miles up the road in Christchurch. So no driving miles and miles for a Fleabay purchase . Just a shame the kits are over 200 miles away now . I knew I should have bought one when they were on my doorstep.
One strange quirk the donor has that has got me baffled is that when driving forward she is absolutely fine but when you reverse her the rear end raises itself slightly and the wheels tuck in at the bottom in the same way that they do when you jack them up. If you pull forward , it corrects itself.
Any ideas ?
I wondered if the rear tracking being out may cause this as both rear axle tie rods that hold the shaft true and mount to the rear of the outrigger , have been bent and straightened at some point.