Your just going to chase the problem around and start guessing by buying parts to fix it.
Drop the pan! Start looking around. Tell us what you see. My best advice is to download factory service manual. Flip through to the valve body replacement page. Ther will be step by step with pictures on how to properly take that thing of and test the electronics. After you get it off you can see the drum, band, strut, and anchor. It will be apparent if somthing is wrong. The best tool you can buy is that factory service manual. Its too hard to explian how to do all the stuff over a forum. Im sure thers a youtube video or writeup on it somewhere if you trust those. Hopefully just a busted band piece. Save you from having to rebuild the whole thing.
It will get worse untill it breaks completly.
If all of this is overwhelming, a good transmission shop should only charge around 200$ or less to diagnose.