May be the rear main seal. Mine leaks less than 1/2 pint annually, but occasionally drips on the front catalytic converter when moving and smells up the cab. A tell tale sign are oil drips on the bottom of the bell housing. I'll live with it until it gets much worse. Just can't see lifting the engine or paying someone $1000+ in labor to replace a $20 RMS. Yet.
Get some engine degreaser and go to the local self-carwash and clean the crap out of the undercarriage. If you are going to be spraying from the topside of the engine compartment, be careful where you spray. If you accidently spray ignition parts you may not be able to restart the truck for a bit to a day or so depending on how wet you soaked stuff.
Once it's clean, wait a few days then look for fresh leaks.
Like @swshawaii said, it might be the rear main seal. I can smell hot oil every time I get out of my truck from where leaking oil blows onto one of my cats. I've tried using at-205, but it didn't work for me. I definitely have oil hitting the ground tho. Might be worth trying, for $20.