Your truck appears to have Rear Wheel Anti Lock Brakes
You can remove the Cluster & have a look to see if the RWAL Module is there
Click Attachment Below
This is how it works:
In normal braking mode, the antilock solenoid valves are inactive. The valves are open allowing normal fluid flow to the rear wheel cylinders.
When high pedal effort braking occurs, the decrease in exciter wheel rotating speed is noted and converted into an electronic signal by the sensor. The signal is transmitted to and processed by the control module. The module determines that acceptable limits of wheel slip/deceleration are about to be exceeded. The module activates the antilock solenoid valves at this point.
The module transmits a signal that activates the solenoid valves. The valves then decrease, or isolate fluid apply pressure to the rear wheel cylinders as needed.
Closing the valves prevents further fluid flow to the rear wheel cylinders. This action isolates the rear brakes from the master cylinder. The net effect is to decrease rear brake apply pressure to the point where the wheels will continue to rotate but not lock.
When rear brake pressure requirements return to normal levels, the solenoid valves are inactivated. This action allows the resumption of normal fluid flow to the wheel cylinders.
The issue you posted almost sounds like it is not being driven enough & the Brakes are basically locking up in the rear, regardless if it is easy (normal) or hard braking (Anti-Lock)
A good Brake Shop should be able to identify the problem in less than an hour