Vehicle Info: 2007 NNBS Chevrolet Silverado Z71 w/ 4.8L Automatic Transmission

Hey guys, I have no knowledge of AT and their inner workings. With that being said, last night I was driving and was stuck behind a row of slowly moving people. I saw an opening and depressed the gas pedal enough to downshift and engage OD but when I did, I felt a pretty solid clunk. I immediately let off the accelerator and pulled to the side of the road to inspect and ensure I had not blown anything.

Upon inspection, nothing appeared out of place. I got back in the truck, started it back up and went to drive off. To get the truck rolling, I had to raise the RPM to above 2,000, as if my clutch was not engaging properly or something. I used my Superchips to return the transmission tune to stock thinking it may have been a faulty transfer, but to no avail.

With what is said above, does anyone have any clues on what might be wrong? I am hoping to get the truck running by this weekend as my girl and I are supposed to be going camping before the temperatures drop much more and I would hate to have to cancel.