Rear Seat Belt problems of the 2003 GMC Sierra

I own a 2003 GMC Sierra crew cab truck. The rear seatbelts lock up and restrain my adult passengers to the point of discomfort. When the passengers try to pull out and release the locking mechanism, the seatbelt ratchets even tighter allowing no movement whatsoever. The seatbelts lock up while driving around town and more so on the highway during roadtrips. The result is that my adult passengers refuse to wear these belts. I have had these belts replaced once already. I took the opportunity to visit several dealerships, and inquired about this issue. I was told that there is a problem with gm's belts, but as of now all the dealerships are doing is replacing the belts for customers, yet the problem persist. It is my opinion that these belts were designed for car seats not adult passengers.