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.

 

Alright, here are some videos that I took today to aid in explaining what is actually happening. To me, it seems that something within the transmission is not fully engaging to turn the driveshaft, mostly affecting first gear (seemingly all gears because of precedence). Any comments, ideas or solutions would be GREATLY appreciated.

WOT in Drive
https://www.dropbox.com/s/pu6dutlqtu...-56-50_823.mp4

WOT in 1st, selected. Acceleration in 2nd, selected
https://www.dropbox.com/s/t7uhy3m402...-59-53_579.mp4

Exterior of 1st WOT
https://www.dropbox.com/s/6mw0ur7u1e...-02-20_785.mp4

Under Truck Exterior of 1st WOT
https://www.dropbox.com/s/w41x4mgxnf...-05-51_595.mp4

Driveshaft Focus 1st WOT
https://www.dropbox.com/s/2mq8f84bsh...-07-35_465.mp4