Shuddering at about 40-60mph?

Hello,

Those transmissions 4L60E has a tendency to shudder at certain speed.
First thing you need to check is the transmission fluid level. If the fluid is low it can shudder. It can also shudder if the fluid is dirty or contaminated.

When you check the fluid, you wan to make sure the vehicle is in operating temperature to get the correct reading.

https://www.2carpros.com/articles/automatic-transmission-problems

If these fluid level is good then it usually requires rebuilding the transmission to get the shudder fixed.

Let me know of any questions.
Thank you.