you can probably do it yourself if its the cloth material that is going bad. just go to a place like Joe Ann Fabrics and buy some material then get some spray adhesive. Take out the headliner and seperate the material from the backing, then put the new material on withe the adhesive. I did this in an s-10 and it worked out fine.
Most of the time the problem is that the fabric has a foam backing and the fabric comes away from the foam over time.
I would just do it yourself Coachpete! As Williams stated it is fairly easy. You can find the headliner material at many fabric shops or go to an upholstery shop or suppler to get it. If they don't have a close color you can easily find it online.
As for removal and prep, just remove the dome light, visor and some trim pieces that hold it up and slide it out the hatch if you have rear hatch. If you have a notch it will go out the door opening, it will just take some maneuvering. Once removed peel off the fabric and then you will have a bunch of foam still stuck to the foam backer board. Take a stiff bristle brush, or a wire brush will work too just be easier with it, and remove the old fabric foam.
Now you need some 3M headliner spray glue, available at the auto parts store, I think it is 77 or 78. Test fit your new material then spray the foam on the backer board, don't spray it on the back of the material, as the can states to spray it on both surfaces. The glue can soak through and leave glue marks on the new material, if you spray it on too thick. Let the glue on the backer board tack up then carefully put on the new material.
From here you have two options. Trim the material with a razor blade at the edges or trim it with 2 inches extra and fold over. I prefer the latter, but it does give a slight rounded look on the edges. I also put adhesive on the folded pieces too.
But trimming at the edge is the "factory way" if you prefer. Then put it back in.
I'd suggest having the old headliner recovered before I'd recommend that. I got a replacement headliner from 50-Resto, and was not real happy with it. The ABS plastic headliner board needed a lot of trimming to fit, and it was hard to cut.
I saved the old headliner and will probably get it recovered this summer. I have the material, and I don't think an upholstery shop charges very much just to put the cloth on the board.