Massive CLUNK trying to go forward after coming to slow stop

I need help with this, not for lack of trying.
If I hit the brakes hard, it doesn't lock up. When I hit them lightly when approaching a car or a stop, or the car wash, the truck locks up and I need to give it a good amount of gas to get it to break free, as if a rope was holding it and then broke. It is a SLAM. It happens in reverse as well.
It happens 50% of the time, then again, I am really mindful of my braking.
When it locks, if I put it in reverse or neutral on a hill, it stops grabbing.
When locked up I have jacked up every wheel and they are free.
I pumped an entire cartridge of grease into the slip joint fitting in the driveshaft. Still locking up.
I haven't greased the u-joints yet. It was getting cold outside.
Any guesses would be appreciated. I am lost here.
Thanks.

 

When is the last time you replaced the rear brakes? I'll bet donuts that they are swelled and grabbing.

 

I will pull the drums. It was my first thought. I thought a frozen e-brake cable but they are perfect.
Only having it for 2 months now so haven't checked them yet.
The swelling makes sense.
Thanks!

 

I had something similar a long time ago. Rear brake would catch on something in the drum and lock up. I would have to pull the wheel to free it and something was on the drum and I had to brake cleaner it... Anyway, this was 20 years ago so memory fails, sorry, but I second a look at drums. Look for gunk on the drum.

 

I bought all the rear brake parts as soon as I got it, but that was because the outside of the drums looked sketchy. I didn't know about the clunk yet.
You guys have given me hope.
Thanks.

 

Update, all of you were right, rear brakes. As it turned out, all of the inner parts were brand new. Swapping out the drums looks like the grab is gone.
Thanks