Based on your description, it sounds like it is going into limp mode. This happens when a fault is recognized by the powertrain control module. Basically, it stays in one gear so you can get it to safety and then checked.
Here is what I suggest: At this point, we need to determine if there is a diagnostic trouble code being stored related to the transmission. To do this, the can-bus should be scanned. CAN stands for controller area network. Basically, all the modules/computers are tied together via a few wires. This type of scan will retrieve a code regardless of the module in which it is stored. Here is a video showing it being done:
I realize most people don't own a scan tool. However, often times a parts store will do it free of charge.
I'm suggesting this because without a code, it becomes a guessing game. Do this and let me know the results. Additionally, I need to confirm the engine size. You indicated a 5.7, V6 engine. The 5.7 is a V8 Hemi. The 4.7L is a v8, and the 3.7 is a v6. Please confirm this for me.
Take care and let me know what you find or if you have other questions.
Friday, September 24th, 2021 AT 8:49 PM