2002 Chevrolet Express rear brake lock up

while applying brake truck pulls to left and sometimes right, rear brake locking up if its raining and I apply brake the abs will release and the truck will stop normally however on dry pavement the rear right will engage harder and pulls truck , I have completely replaced all components including bearings and races wheel cylinders front calipers and rotors pads and shoes and front brake hoses, have had several shops inspect and still can not locate problem. Truck is a dual rear wheel and has a utility body, weighs in at aprox. 9500 lbs