What sort of wobble is it? Like from the side the cogs are not spinning quite on center? Or when viewed from the rear the cogs are oscillating side to side? Also give us an idea of how much it is. Like the thickness of a business card or something more or less?
Is it actually loose and wobbly or is it firmly in place but has runout when spun? If it's a freehub and if the whole thing is free wobbling then it might be the hollow tubular "bolt" that secures the freehub to the wheel hub that has come loose. And that compounds things and will also show up as a wobbly wheel since the drive side bearing cup for the axle is part of the freehub. Look at an exploded parts diagram if not sure.