I have just left. About 3 yrs prior to leaving I was sent a "last tour of duty request form", not sure of its real title, which I filled in and returned to PMA. Not sure if it affected anything, however I did stay at my requested unit until starting my terminal leave.
Have a chat with your desk officer, or his pa.
However, I was totally upfront with my desk officer about the fact that I would be leaving at my my 16/38 point, so it could have been that he just wasn't arsed about posting me.