sounds like a headgasket to me if you are 1 to 1.5 inches over the full mark and the oil is a milky brown.the mid 99-01 heads was a weaker head casting that was prone to cracking.they mostly cracked beween the 3rd and 4th exaust port but can crack other places as well.if that is the case dont pick up another head in those years.instead pick up a head from a 96-98 4.0 they are stronger and the exaust ports line up better for better flow.
drain your oil and replace it as well as the filter.pour in 6 quarts of oil and let her rip.if it goes over full and turns milky again i gurentee it is a head gasket leak or a cracked head.