In case you stil have those Xbox controllers lying around that drift. this is a pretty good video that goes over correcting the issue. Have not tried it yet but might give it a try one day.
Seems very involved. not an easy fix.
Way above my pay grade
Trade the fucker in at Gamestop and get a brand new seems easier to me .
I had similar problems with my 360 controller. I just opened it up and removed the stick, cleaned it and put it back. Right now, my xb1 is drifting a bit and the left dpad key needs to be hit too hard for it to work.