Brian,
I worked for GMC many years. I have never heard of a trans slip on deceleration. How can a trans slip on deceleration? Not in my book.
A trans slip would be, when the rpm's go high and your vehicle is not accelerating as it should.
This is why you have not received a response in this group.
Please describe the what you are experiencing. Perhaps you might get a response then. Give details.
GMdude