I am assuming your 400 has permanent 4WD as I think the switchable one came in with the circa 2003 Foreman 450. If so, the splines will have gone on the front prop shaft. Can be at either end, water gets in and rust starts, then the splines wear away. Take the mounting bolts out, slide the front diff forwards, and take the shaft out. It is just short enough to come out with the muff coupling compressed.