I went through a lot with mine. In the end, getting that 1/2" spacer was the easiest solution to the problem. If you know what you're doing it takes less than 30 minutes to install with a manual spring compressor. With an absolute empty tank of gas there is not enough height difference to notice, and I rarely run with less than 2-3 gallons of fuel left anyway.
Here's a recap:
Front measurements: even
Axle measurements to ground front and back: even
Body measurements to ground: 3/8" - 3/4" difference depending on fuel level
Frame to ground: 1/2"
Frame to axle: 1/2"
Spring lengths: even; same part number
Shock lengths: even; can't remember if they're the same part number
Swap shocks: No difference
Swap springs: No difference
Fully loaded bed (1000+ pounds): even measurements
Trying to remember, but I think with springs in and no shocks the measurements were even with unloaded bed.
Lifting the driver side of the bed and tightening the track bar put the truck at the right height until it settled again. This leads me to believe it's not a frame issue because the front measurements never changed. The frame is supposed to be solid, so if the rear changes the front should change and vice versa. That never happened. Once I got that out of the way I focused solely on the rear, but without taking the entire thing apart (including track bar and links) I couldn't find the cause.
None of my friends or coworkers have a Rebel, so I couldn't grab their shocks to have a true comparison. My friend's ORP Laramie is even measurements on both sides, but he measured it not me.
I was just wondering, has anyone measured the frame to ground distance to see if the frame is level? Could be a frame issue(twist), or the body mounting, or like 
