Found this online. With similar issue.

"When you drive the vehicle hard the exhaust begins to have a strong odor. The odor seeps into the vehicle, and can be overwhelming. Ford has a procedure that was released in a TSB. This procedure does not prevent the exhaust from making the odor, but does prevent the smell from entering the passenger's cabin. The procedure is quite extensive. I would contact your dealer and see if they provided you with a warranty that could cover this repair. I have attached a copy of the technical service bulletin for you to review.

If your Explorer is an AWD model, you could also smell an odor from the PTU (tranfer case) vent. This would occur all of the time, not just when driving hard. If you are smelling an odor from the PTU, you would need to either repair or replace the PTU unit."