F150 Transmission- White "bobber" with stem and O-Rings
 

I'm changing my transmission fluid and filter on a 2001 F150-SCrew, 5.4L. When I took the pan off, I found a white "bobber" w/ stem and O-Ring arond the bobber...approx. 1/2" in diameter, sitting in the bottom of the pan. Any idea what this is and where it goes?

That is from the assy plant it seals the trans till they put in the dipstick, just discard it.

He's correct, just toss it in the trash. It cost more to remove it than to leave it in the trans so they just leave it in there until someone opens the pan and services the trans.