Due to someone else's advice in this thread, I did a hard reset of the car (left wheel button + hazard button for 15 seconds), and it has resolved the issue.
I can still feel a very slight engagement of the second motor if I'm really paying attention, but I'm not getting that lurch & thunk...
I am experiencing this issue now.... first noticed on a trip to montana over the mountains.. It's very, very noticeable going up a highway pass.
Rivian wants me to take to the SC, which is 300 miles away.
Definitely a bit frustrating that a software update messed up the drivability of the...