Hello and welcome to JustAnswer!
Is this something that happens as you are slowing down? What speed does it happen at?
WHEN SLOWING DOWN, ALSO IF SLOWING AND THEN ACCLERATE IT HAPPENS THEN ALSO
What speed does it happen at while you are slowing down?
DIDN'T PAY THAT CLOSE ATTENTION, BUT I WAS SLOWING DOWN TO MAKE A TURN
I'm assuming it was a harsh 4-3 downshift, which would happen as you pass through about 30 mph. This is caused by the clutch volume index for the underdrive clutch being too high. This isn't an uncommon thing to happen after the trans has lost prime, such as dropping the trans pan to change the fluid.
When you start the engine the trans controller is watching the pressure switches for the different clutch circuits. If the switches don't close right away it can cause the clutch volume indexes to start counting up. A higher clutch volume index will cause the TCM to apply more fluid to the clutches at shifts, which makes them harder. The only shift where you can feel the underdrive clutch come on is the 4-3 downshift, which is why this is most prevalent there.
What you'll need to do is have the clutch volume indexes reset with a scan tool using the "quicklearn" procedure. This can be done with a dealer scan tool or possibly some high-end aftermarket scan tools. It will only take a couple of minutes to do.