Take a look at the ABS control module. I have never had this problem with the runner, but I definately had this with my volvo sedan. It would drive fine, then slip out of gear and the engine would rev. I had to pull over, turn the car off, and then when I restarted it, it would run normal for a while. Then it got to the point where it would not go into reverse. Also the speedo did not work after stopping at the store and turning the car off, but upon restarting the vehicle it came right back on. It was a short in the ABS control module.

I am thinking the problem is something electric, especially if everything with the tranny fluid is normal and has been.