Mine were hazing, and turning yellowish from UV exposure (looking horrible), and so I went to
www.fordpartsonline.com and bought 2 replacements at the time. Yours are shown here:
http://www.trademotion.com/partlocator/index.cfm?action=selectTree&CatalogID=2&sit eid=213671 (by the way, click on 'front lamps', then '98-00', and then 'headlamp components')
That is the best fix for this problem, because sanding and stuff will just remove the special hardened protective clearcoating on the old units, and then they will be unprotected from dings and chips (only compounding the current problem). After I put new ones on my car, it practically looked new again. (nice!!)
PS-
I ran those new ones for a couple of years, and then wound up hacking them all up anyway to install the new xenon projectors (
seen here )