Shudder in reverse

2011 Sienna, 198K miles. It recently started giving me a shudder in reverse but only on an incline. One mechanic just said to replace the entire transmission, which would be 6K. I was hoping to get a little more life out of my sienna, but that's just too much to spend. Anyone remedy a similar issue? Would a full transmission flush do anything? Planning on getting another opinion, but we are an hour away from a dealership.