You can find STP at your local hardware store. It's used for deck cleaning, siding, etc.
It has "phosphates"....the stuff that was banned from household detergents many years ago due to excessive algae growth in the water system.
If this doesn't work, you likely won't recover these shorts to the pristine white. The optical brighteners may be gone.
Head IG Radical MP
Mosquito Bite @ 50#