I am not sure if it would tell you anything since there are not any lights on, but it can't hurt to try.
Also I googled tranny problems, and although this is a slightly different presentation of symptoms, this guy had a problem with his tranny that was Battery related??
When was the last time you had your battery replaced? Or even tested at a napa/auto zone?? Worth a shot, and cheaper than a new tranny??
2000 4Runner Automatic Transmission (Battery???) Problem - Transmission Discussions at Automotive.com
I am still thinking electrical, assuming that you checked everything that Dr. Coffee just said.
