Transmission Gear Slipping problems of the 2011 Toyota Sienna

I was driving the vehicle down the interstate and it started to slow down. The engine was still running. However, the transmission was slipping and I could not maintain the posted speed limit. The only way to keep the vehicle moving was to rev the engine up to between 5,000 and 6,000 rpm. Fortunately, I was able to get to an exit and get the vehicle to a parking lot. At that point, the vehicle would not move under it's own power. The engine would run, but there was no power to the drive wheels. This concerns me as I was lucky to get the vehicle off the interstate and off the road without incident. Otherwise, given the traffic and posted speed limit, I was concerned that someone would run into me.