Front. IF it's the splash guard, fixing it depends on what exactly is wrong, may be a simple bolt/push plug has come out, maybe the plastic cracked. Have to see it.
Are driving over this dip in the morning straight on or one tire at a time (like driving over a speed bump and only having one tire go over the bump)?
Before you leave in the morning push on the corners of your car in an effort to cycle the shocks/struts on each corner and see if that changes anything. Maybe it's something thats needing lubrication and sitting over night it settles and that dip in the morning frees it up. Just guessing and throwing crap out there :)
2014 Focus SE Manual -
2006 Focus ZX4 SE Auto -commuter 110k /traded