99 silverado clunk

Hello, I have a 99z71 with a 5.3 in it. Once in a great while it will give a clunk shifting from 1st to 2nd gear. Mainly happens when a/c is on and car has been running for a while. It only happened once maybe every 4 months so I never took it in to get checked out. This past weekend at a stop, my check engine light came on and it clunked in every gear. I was stuck in stop and go traffic so I experienced this a great deal. Seem like it hesitates to go into gear and when it does it clunks and shakes the truck. The next day It was fine. Im wondering if i should take it too a tranny shop or to get a scheduled maint on it. The last maint I had was at 60k. I have 110k now. The oil and air filter are the only things I have changed.

Sorry for the story and thanks for any help

Since it's noticeable more with your a/c on, it could be more of a engine problem than transmission. The a/c compressor will pull considerable HP from a poor running engine. Your "Check Engine" light shows that the computer has stored a code. Have it scaned and maybe it will give you an ideal of what's going on.