Thanks for the input everyone. Updating this incase anyone out there might be in a similar situation. I took my 5.3 to a mechanic I've had success with (I don't trust the dealership like I do this local mom and pop shop) and ended up doing a complete fluid flush including pan drop and clean, new gasket, new filter etc. After driving my rig highway, up hill down hill and side street, I can confidently say she shifts just as good if not better than she did when I first got her. My initial concern was that the truck's transmission was at the point where flushing might do more harm than good. That said, her past owner seemed to have taken good care of her, and I think its worth mentioning to anyone in a similar situation that I asked my mechanic to flush the transmission via hand/ transmission pump, and not anything high pressure. Fluid-wise I went with standard ac ACDelco Dexron VI stuff. Very pleased with the outcome despite her mileage teetering in that gray area of flushing vs. pan drop.