Hey guys -
So after a week of troubleshooting, I finally stumbled upon the answer. Loud cockpit noise. So I went to Walmart, bought a pair of $10 "leather" gloves, cut off the forefinger, punched a pin sized hole, put it over the mic, and zip-tied it down. Now ATC has no problem hearing me! Just flew down to LA tonight with no issues.
Things that were preventing me from diagnosing this sooner:
1) Better reception in close proximity to airfields.
-I would assume that every time I tried it with an airfield I was usually below 120 as opposed to 190 when cruising around. I guess that squared velocity term in dynamic pressure makes for a big difference.
2) No difference with headsets.
-Just thinking about it now, if the ambient cockpit noise is loud enough it will affect all mics equally.
I feel kind of dumb that it took me this long to figure out, but I appreciate all of your help! Now to figure out how to seal the cockpit better...