Is it hard to fix a sensor which makes it leak oil in the front area of vehicle

I Have A 2001 F250 Ford Super Duty & It Leak's Oil in the Front A lot Of Oil.. I just Notice It the Other Day Ckin My Front Tire's & Saw it, Anyways I Thought it Was The Trany & Had it Cked Out & They said It was A Seal, Cuz I Already had the trany Rebuilt.. But It Wasn't The Seal Cuz it Still Is Leakin A lot Of Oil, He said it Had To B Up in The Motor Like Somewer Around the Oil He Called It Somethin But I Can't Remember Now.. But I KW He Said it Was Up ther In The Motor, Like Somewer Next to The Oil dipstick??? & That it was A Plastic Box That Usually Goes Bad & Crack's...Do U KW What It Could B??? Plz Help If U Can!! Thx