'Eastbound & Down' to Lindsay Lohan -- You're F**kin' In!

They both experimented with drugs ... they both torpedoed their careers ... and now Lindsay Lohan and Kenny Powers are joining forces on "Eastbound and Down," TMZ has learned.

SPOILER ALERT: sources close to production tell us, Lindsay signed on to play the adult version of Kenny's illegitimate daughter on the HBO series finale ... and she's currently filming on set in North Carolina.

Makes sense Kenny's kid would become Lindsay Lohan. If you recall, Kenny stashed his bong and all sorts of sex toys in the room of his infant son's room at the end of last season.

It's a pretty big opportunity for Lindsay -- her first real acting gig since she left rehab -- because if she's funny, a spin-off could be in the cards. Crazier things have happened.

Now here's a clip of Kenny Powers dancing his face off on ecstasy.