Well, no sooner did I post this....and then I figured it out.
In the case of the gradient fills that didn't work when printing/exporting, I was applying an Opacity setting to the color values......for example, I would start the gradient with "Red" @ 0% opacity and end with "Red" @ 80% opacity.
What fixed it was leaving the opacity setting at 100% and using the RGB Sliders to specify the starting and ending colors in the gradient. Much more difficult than simply using the same color with varying opacity but the one gradient that I changed exported to PDF correctly.
Interestingly (or not) any SOLID color fills using an opacity setting < 100% printed and exported just fine.
Sounds like a bug in Numbers to me.