Fixed it! It was a short to the RF Shield Can. I was a little over zealous with the solder.
First, I lifted the micro pin, and the trace was still shorted to ground, but the pin was clear.
Next I wanted to lift the modules pin but it's a lead-less package (rf module on a pcb with pads on the bottom. So I couldn't do that (or remove the module easily).
So I then cut the trace, the short was still on the module side. I wasn't too happy, I didn't want to have to remove the whole thing. But then I thought lets just try wicking away solder from the modules shorted pin, maybe I would get lucky.
And the short cleared! The solder must have crept up the side wall (it's like a via cut in half) to the can causing the short.
After putting on a bodge wire from the micro to the module my UART connection works like a charm.
Now I know to be a little more careful when hand soldering rf modules with cans. And I have a new test to run (check for ground shorts on the RF module pins).