1999 DODGE RAM 2500 SERIOUS BRAKE PROBLEMS?

My 99 dodge ram 2500 has some serious braking issues. I had a total brake job done 8 months ago. New drums and shoes, new pads, rotors, and calipers. My problem is that the rear drums feels like there sticking. As soon as i pull out the driveway and barely tap the brakes, the rear lock up. And as I drive the rear just stays so stiff when I stop. It's embarring when I go to stop it looks like I'm just hammering down on the brakes. I have to adjust the rear twice a week to loosen it up just to drive it. I took it to Dodge and they said first the abs module, then try something else, and if that dont work then the proportioning valve. Seems like a lot of money just to throw at something that dont fix it. If I do hit the brakes hard enough I can feel the abs in the rear kick in. It does seem like all the pressure is going to the rear to stop me. Is this the proportioning valve causing this? If so how much does it cost? Any help would be helpful.....