Do you have rear disc brakes or drums? Is one tire locking up or both? If one tire is locking up then it is probably one of three things:
1. your parking brake is stuck on one side (doubtful)
2. something broke on one side rear (could be a spring if they are drum brakes), this could cause more pressure to go to the other side and lockup the brakes.
3. bad wheel cylinder (drum) or caliper (disc) on one side
I'm new to dakotas so I don't know what style rear brakes you have, but something is worn or broke if you are locking the rear brakes. The only other reason why your rear brakes would lock up is if your front brakes weren't working.