<<I measured at the radiator (top corners) and they are running about
the same temps. If anything the van that runs hotter has a slightly
lower temperature at the radiator>>

And here is the answer.

If the water isn't circulating through the clogged radiator it'll never
heat evenly.

Take the palm of your hand and put it at the top and then the bottom of
it.
You'll notice a significant difference in the temps.

Replace the radiator.