Rear wheels intermittently locking up

I first noticed the problem when I was coming to a stop at a stop sign. Right before my truck came to a complete stop there was a very violent shudder (like going to fast over a cattle guard). It was brief, and when I accelerated from the stop, everything was okay. The problem persisted, but intermittently. It didn't happen at every stop, and interestingly, it only happens when coming to a slow stop (like riding the brakes to a stop). It does not happen when coming to a more forceful, abrupt stop. It also does not happen at any other time when braking. The problem worsened by the fact that after shuddering to a stop, my truck wouldn't move forward after letting off the brake. It seems like the brakes are stuck (if it is the brakes). When stepping on the gas to accelerate, it feels like the brakes are stuck, and it takes more gas pedal to move forward. Perhaps important, the brake pedal does not get hard when the sticking occurs; it moves normally. What's even worse is that when I give it gas, the truck violently shudders (like it does when coming to a stop). So, I try not to give it gas when it happens. Eventually, whatever is sticking becomes un-stuck (on its own), and everything is back to normal. Sometimes it takes a while for it to become un-stuck. Again, I want to mention that it doesn't do this consistently; however, it does do it most of the time only when coming to a slow stop or riding the brakes to a stop (like in a drive-thru line). The problem does not happen when going in reverse. Initially, I assumed it was the front brakes. Honestly, when the truck started shuddering, it wasn't obvious that it was the front or rear (it is a bad shudder). I replaced the front pads, rotors, calipers, and thoroughly bled them. Of note, the brake fluid looked awful. I did a test drive, and unfortunately, still have the problem. When pulling back into the driveway the shuddering and locking occurred, and my dad was standing next to my truck. Based on what he saw and heard, it is coming from the rear. As a matter of fact, at that instance, the rear wheels locked to the point where they dragged to a stop. It appears to be occurring at both wheels, as there is no pulling to one side. Of note, after working on the fronts, the problem seems to be happening less frequently. I'm not sure if that means anything or not, and maybe i'm just kidding myself because now I only drive it when troubleshooting. Also, the brake pedal has a lot more travel in it now to stop. My dad and I thoroughly bled the fronts, so I do not think there is any air in the line. Sorry for the lengthy post, but I feel like this a unique problem (maybe?) Because of the shuddering, and it is on both rear wheels. I have spent many nights scouring the internet to find some help with this problem, and unable to find anything similar, I finally decided to try a post. What I'm trying to avoid is replacing every component of my brake system, and that possibly not even fixing the problem, because it might not even be brake related. Thanks for reading, and any suggestions or help is much appreciated.