I feel your frustration buddy - my Car has had a Good shot at testing my patience this year
(although Now it's not the Car at Fault - some tarry fingered Swine has stolen my tools from a Hotel where i was doing work, leaving me unable/unwilling to continue working there and currently unable to take on other work
so looking for temp employment again to fund the last bits I need to get my Car BOTR
what can you do.....you've got to laugh)
I've no experience of Autos or Gearbox Issues - Could it be as simple as a sticking Rear Caliper ?
causing more resistance in reverse resulting in a "Jitter"
just a thought - I hope you get to the cause and it's nothing too drastic
Paul.