You have a few options:
I have heard of baking composite headlight housings to get them to pull apart. Peel the plastic off, then polish both sides thoroughly, then glue them back together.
Shop eBay for new / like knew / or aftermarket replacement headlight housings. You can get some decent deals on eBay. I got a set of lightly used factory tailights for my Tahoe for only $45 shipped. Just got to be patient and shop some.
Go to a junkyard and see if they have a your vehicle in stock with intact headlights, if they do, perhaps they will be in better condition.
Highly recommended - replace your factory bulbs with an HID kit. I just outfitted HIDs to my Tahoe for only $60 and the difference is stunning. I used to have SilverStars in my low beams, which I really liked. I put 5000K (color rating goes by "K", lower the number, the more yellow the color, higher the more blue, 5K is a very pure white) HIDs in and they made the SilverStars look like little flashlight bulbs. Just amazed by the difference. I think the HIDs would help punch through any yellowing from age.