1999 Chevy Suburban Driver's side rear wheel seizing

Are you sure the primary was on front and the secondary was on the rear when the shoes were replaced?

Well you wouldn't have to take it completely apart, just the wheel and drum.

I don't expect a proportioning valve to be the problem.
Most RWD vehicles have one main line going to the rear brakes, the T off to each side. So equal pressure would be going to both wheel cylinders. (Although I do not have a hydraulic schematic for your truck either).
I could see a smashed line going to the other side keeping fluid from getting to it, and then too much fluid going to the side that's locking up.
Do you remember if both sides were wearing about the same?

Might try putting it on jack stands and run it in gear. Have someone slowly depress the pedal and see both rears will actually stop turning.

I just can't get the idea out of my head that the secondary is on the front and causing it to over-energize.

Give me the last 8 digits of your VIN.
You can send it to me in PM if you are not comfortable with posting it on the internet.

PS, I have to leave in 90 minutes to watch my son while my ex goes to an appointment.

I want to explain why I need the last 8 digits of you VIN.
Back in the old days when we used parts book and micro fiche to look up parts.
We had to ask all sorts of questions.
Say you wanted the Yoke on the front of a drive shaft.
If you had an automatic, you needed page I-17,
If you had a manual, you needed page G-20
If you had and extended frame and a 2-piece shaft, then you needed page H-12.
So if I (the parts guy) didn’t ask the right questions, or know where to look, I could possibly sell you the wrong part.

The new computers and parts programs take the human error factor out of it.
If you have VIN 123456, then the computer knows you have an Auto with a once piece drive shaft and thus only give me access to the correct page.

So the VIN is the “key” to unlocking the parts program.
Without it, the computer is just a dumb box. And will not let me look at anything.

I want to pull up a picture of your brake plumbing.

I've looked at the piping for your rear brakes. And they are setup as I described in the previous post.

I cannot make any more recommendations until you do some testing / inspecting.

I'll keep an eye out for your update.