The dealer only replaces trannys, it's in their (m
best interest to sell you a new/rebuilt tranny. See what the other shop says, you may want to take it to an indie that has the computer to connect to your car. While you are there have them check the fluid level, maybe drop the pan, replace the filter and replace the ~3 quarts that were in the pan.
You could have a GM tranny.
What year and model do you have?