Transmission is stuck in third gear and neutral light blinking

This is called limp mode and more then likely is an indication of internal transmission issues or possibly a sensor issue.

Here is a guide that talks more about this:

https://www.2carpros.com/articles/reduced-power-limp-mode

Along with this issue is most always a couple DTCs so we are going to need to scan for codes and find out what is there.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Once we know what codes we have we can make a better decision but how long ago was this replaced? Also, I assume when you say it was replaced with a rebuilt unit, that means you had a shop install a used unit that they rebuilt or did you buy a remanufactured unit from a company and install it?

Let's start with this info and go from there. Thanks

Ugh. That is the worst. Do you have a copy of the work order that they gave you showing what they did or replaced?

Also, what did you take it into the shop for?

Clearly we need to at least contact them and alert them so they have an opportunity to fix it. The longer you take to contact them the more opportunity they have to say it is not related to what they did.

The most common cause of this is a valve body or electrical connection/sensor issue.

Please let me know if you can at least get the codes that are currently in it and that will give us a good idea if it is related based on what they did. Thanks