Problem With 97 Sierra

This Only Happens After The Truck Has Been Sitting All Day Or All Night. When I Start To Back Up I Can Move Like 5 Feet And I Get Stopped Dead In My Tracks. but Then I Can Put It In Drive And Pull Forward And Then Back Up And It's Perfectly Fine. It Dosn't Make Any Noise Or Anything. It Seems To Me The Back Brakes Are Locking Up, What Do You Guys Think? What Do You Think Might Help The Problem?