I am not an expert on this, but Googling shows P0770 as Shift Solenoid E Malfunction. And, one page says it is the solenoid which locks up the torque converter.
My 2002 Sienna has nearly 165,000 miles on it, and if it fails, I expect to take it to a good transmission place for rebuilding. It should be much cheaper than buying a rebuilt from Toyota. (I am assuming that is a rebuilt, since with labor a new one would probably be a lot more, but you did not say -- and I am not an expert as I said.)
Scout the Mechanics Files somewhere on this URL for a good transmission rebuilder near you, and ask them what it costs to fix this problem.
A lot of people believe it is much cheaper to go to an independent mechanic, if you can find a good one. I go to the dealer only because in my community the really good independents are so swamped they want to keep your car for several days and that is not practical with only one car.