**Update** I went back to the transmission shop and they are standing by their statement of it not being a transmission issue. They went as far as hooking up a shift sox directly to the transmission to manually shift the Yukon and they said it shifted perfect. For those that don't know, the manual shift box plugs directly into the transmission taking the "Yukon" out of the equation. After this test, the transmission shop and the mechanic they referred me feel it is more than likely a bad ECM. They want me to go to the dealer and pay them to diagnose the issue. I am not a fan of going to a dealership but this may be my last resort.