It seems crazy but the mileage is stored in some sort of memory module inside the cluster. I've never heard of anyone adjusting the mileage on a used cluster that they swapped into their truck. (this is not to say it's not possible, I've just personally never seen it done, I'd inquire at a dealership to see if it's possible...although I think it's not probable due to people abusing that power...but again, pure speculation on my part, and wouldn't hurt to explain your situation and ask)To remain legal a replacement odometer must reflect the actual mileage. Specialty speedometer shops (and presumably Toyota dealerships) can handle this if it's necessary.
The odo function is never really addressed in the factory documentation, and the odo isn't mentioned or illustrated in the Wiring Manual. Because of this I'm uncertain as to where and how the actual mileage is stored.
01-04 clusters are digital with a backlit led displayPretty sure they are still a rolling display (analog) and therefor only stored in cluster. I am interested in this as it may be a more cost effective way for me to add a tach and keep that original look (don't need a 5 inch racer boy tech but knowing rpm would be nice).