I had a similar issue a few years ago -- don't panic, it may be something simple. Mine would appear to drop into neutral between shifting from 1 to 2, then S L A M hard into second.

Get yourself a can of DeOxit and a flashlight, and maybe a mirror or your cameraphone.

Disconnect the battery before you begin. This will allow the ECM's to reset anything they may have learned, and plus you're working with electrical stuff so it's a best practice.

Crawl under the vehicle under the driver's side, just around where the pedals are. Look at the side of the transmission and look for a white plug coming out of the transmission with a cable going upwards. CAREFULLY flip the lever and disconnect the connector, and pull it upwards. Look at the pins and see if you see a bunch of corrosion in there. I bet you do. This is where the transmission ECM connects to the valve body and tells the transmission how to do things. If you have corrosion, they're not talking back and forth very well. I will never understand why Toyota put such an important connector in such a vulnerable place.

Take the DeOxit spray and blast out the plug and the base. Wait maybe 20 minutes, then do it again. The pins are fragile, so don't go in there with a wire brush or a dremil to clean them.

After 2 or 3 rounds of spray-n-wait, let the pins dry for about an hour or so. You want it all to evaporate and leave the protectant behind. Smear a little dilectric grease on the seals so when you reconnect the connector, it keeps moisture out.

Reconnect everything and go for a ride. If everything works well, congratulate yourself for not spending thousands on a replacement transmission!

Let us know how it goes so the next person can search-n-learn.