85 C10 pickup

[PolarBear]

I recently worked on my friends truck. I did a bunch of stuff as it sat in a field since 1996 or so. Rust wasn't too bad surprisingly.

One issue I couldn't nail down was a shake from the rear end while driving. Turns out this was an issue before it was even parked, his uncle told him at a family function, he was the previous owner.

We put new shoes and, most, of the springs in the drums, both sides. The spring kit didn't have all of them to replace. We also put new wheel cylinders in. I had originally put his new shoes in and used the old drums. I checked the curvature of the new shoes to the old ones, but the shoes weren't original. I didn't think to check the shoe to the inside of the drum at the time. The problem persisted and we put new drums on it too. I also replaced the parking brake cables to the drum, but we kept the front cable to the pedal as that one wasn't rusted and locked up.
I adjusted the shoes so they just started to drag when spinning the drum. For the parking brake I had it so it just took up the slack in the pedal.

So the issue is that when you drive the truck shakes violently in the ~25-40mph range from the rear. If you keep your foot on the gas and lightly apply the brakes or parking brake the shaking changes, so it is definitely brake related.
What gets me is that the problem is intermittent. When I was driving it around in the neighborhood it was fine. I put 30 miles on it at low speed without it shaking. Then I drove out to Hanlon in PA and it was fine when I started, then progressively got worse. When I got there I loosened the parking brake so much that it would engage, but it would take ~3/4 of the throw to engage. If was after this that we put new drums on it. I drove it around the neighborhood a bit more, no problem. Then I drove it the 30 miles to his house down route 33, no problem. Then he drove it around the neighborhood the next day and it started shaking. Then he tried it again a couple days later and no problem. Couple days after that, problem.
Truck only has ~46,xxx original miles

I had some recommendations to check the U joints. I thought about it, should check the runout on the axle flanges. All the rear leaf bushings should probably be checked.
Any other ideas?

[sweetbmxrider]

Tire pressure, balance, and belt shift etc

[PolarBear]

Wheels/tires are all new and balance out on the machine. I also don't think that would account for the issue being intermittent, but not positive on that

[V]

Could it be a heat related issue where something is expanding then causing the issue? Possibly something with the brake setup? Maybe a sticking wheel cylinder?

[sweetbmxrider]

Greased all contact surfaces and nowhere for something to get hung up on after braking?

[PolarBear]

Originally Posted by V

Could it be a heat related issue where something is expanding then causing the issue? Possibly something with the brake setup? Maybe a sticking wheel cylinder?

It kind of seemed to be like that initially, but he drove it on a cold day too and it still did it.

Originally Posted by sweetbmxrider

Greased all contact surfaces and nowhere for something to get hung up on after braking?

What would I grease in a drum? all the parts were dry when I took it apart. All the drums I have worked with are dry inside when I have opened them up.
My friend took it to another shop and they supposedly "lubricated" one side, and it still does it.

[sweetbmxrider]

Anywhere the shoe contacts the backing plate and the pivot point opposite the wheel cylinder. Usually you can see where the paint has worn on the backing plate, just a dap of grease there.

[PolarBear]

My friend took it back to the shop that fixed the other stuff. They changed the U joints and cut the new drums. Seems to be OK now. I hope that is all it was