I have a 2000 Express 3500. 180,000 miles tranny replaced about 20k ago. It recently developed an odd condition... during the 1st 10 min of driving it will make a deep knock somewhere behind me in the drive train. It doesnt happen often but it usually will happen at parking lot speed or around 45 mph. I also noticed it when turning a sharp right in the parking lot. It all seems to go away after about 10-15 min of driving. I checked the u-joints for play I checked my differential fluid and it seems fine. I have had a bad spark plug on a ford truck do something similar but this hesitation or knock is very irregular and the ford did it constantly. I have purchased new plugs and wires but am waiting to dive into that project till I do a bit more research. I left it parked since I noticed the issue because if its a tranny or other issue i dont want to make it worse.