Brakes squeak in the back?

I am a mechanic, but I am at a loss, we replaced the rear brake shoes and equipment due to grinding in the back. We fixed the grinding issue but now the vehicle will not move when shifting into drive and/or reverse unless gassing it and then it sounds like a train from how bad it squeaks. So, we replaced the shoes once more just to make sure of any faulty equipment, the squeak went away temporarily and then came back all over again, so now we have replaced the wheel cylinders and adjuster pins on both sides, squealing was still there but not as bad as before but we let the truck sit for an hour or two on a flat piece of concrete and went to move it and it seems the squealing is just getting worse all over again. Any ideas? I think it may just be the shoes wearing in, but the noise is awful and makes the vehicle feel unsafe to drive.