Ok was making a little grinding noise for about 2 day when braking, and thought ok going to change brakes got home and put it in park, then when tried to reverse the whole back end was locked up. Tried it in 4 wheel drive same results, rocked it gently back in forth and it came free but my brake pedal now goes down to the floor. Now I have the esp, bas light. The red brake light on the left and the yellow traction on the right. But why and how would those lock my rear brakes down hard enough not to move my truck? And where do I even start to fix this? Any help would be greatly appreciated

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Pull the wheels, check the brakes. Also check the brake lines.....