Well if neither of the rear disks is working then you know the issue has to be between the master cylinder and where the brake lines split for the rear calipers.
I assume the master cylinder worked before with the rear drums. It should still work with the rear disks, if anything it would have a residual pressure valve for the drums that could potentially keep the disks locked up. So I wouldn’t suspect the master cylinder since it worked before.
Was the car disk/drum or drum/drum originally? If it was drum/drum you should have a distribution block, not a combination valve. If it was disk/drum or if it has a combination valve it may need to be “adjusted”, which is done by bleeding the brakes in order- right rear, left rear, right front, left front. There’s a procedure in the FSM. That can recenter the bias adjuster in the combo valve.If it was drum/drum and you have a distribution block it’s unlikely to be the block. Not impossible but unlikely, especially if it worked before.
It could be the brake hose from the frame to the rear axle. In fact that would be my first suspicion. With an 8.8 swap that’s probably a brand new part, but, that doesn’t mean a thing. The first thing I ALWAYS suspect if both rear brakes don’t work is the frame to axle hose.
