What is causing a transmission shudder when shifting at low RPMs?

Perform a transmission service with Motorcraft brand mercon V transmission fluid and a new transmission filter. If you find a little plastic plug in the transmission pan when you lower it, don't be alarmed because that is the original dipstick tube plug that gets pushed through during the assembly process. This was part of an old technical bulletin that we got at the dealership level back in the day, and it worked as long as the transmission was healthy.

Here is a guide that should help:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission