Removed the front cowl (I think that's what's it's called. It's the the slotted panel that's right below the windshield) last night & wiped it clean. No dead animals, just some mud. Nothing that looked like it should have caused smells.
After cleaning, blasted half a can of Lysol down the AC/blower air intake with the AC running & blower on it's highest setting. Sprayed some Lysol down the vents from inside the Jeep too. The Internet says that it should kill any mold/bacteria that may have built up inside the airbox/condenser/vents etc..
Removed & threw away the in-cabin filters that I installed just a few weeks ago. Were still new, no smell in them but figured I should start from scratch. New filters are on order.
Result - it now smells like a freshly disinfected bathroom. No stale smell on my shirt when I got to work this morning...which is positive.
Need to monitor for the next couple of weeks to see if this is actually resolved. I'm hoping to God it is!!!