I had the exact thing happen to a pickup, and I mean exactly to your description. I found it to be a large snap ring, I guess you'd call it, that held the higher gear clutches on the shaft in the transmission. I dont really know what all the parts are called I had someone do the work. The trany was completely aprart to find this out, so we rebuilt the whole thing. If it would have been a standard I'd have done the work, but I'm not sure about an automatic. This may not be your problem, but they sound the same.
Have a blessed day !