I have a 1998 Chevy 1500 with a 4L60E transmission. Truck has almost 155,000 on it. Ever since I bought the truck 2 years
ago it has made an intermittent clunk when it shifts from 2nd to 3rd gear. There may be a 2 or 3 week stretch where it
wont do it and then it might do it for a week straight after that. Trans fluid is good and not low. Could this be something
where the bands need to be adjusted? Not awfully familiar with automatic transmissions.
Some Chevy transmissions are known to clunk at times. Have you checked your u joints and transmission/motor mounts. Might be something as simple as a bad mount bushing.