Take it out and get it recovered. The material is foam rubber backed and they don't last forever. Depending on the size, usually run about $100 to get recovered, a little more being crew cab. The headliner itself is a formed cardboard type material, so be cautious not to bend or break it,
I have a '92 and the headliner was sagging when I bought it.
I found an illustrated tutorial online that explains how to remove & replace the headliner.
I made the STOOPID mistake of re-using the original fabric :facepalm: and now it's sagging again. It's too late in the season right now to remove and replace with new material so I'll wait for spring and put in something with a really bizarre pattern.
I have found that fabric stores have a limited color supply of headlining material. I am sure it is not that much in demand through that retail establishment.
A better source of material with additional color choices would be available through the internet online. Do a search on auto headliner material. You might want to recover the sunvisors at the same time to make sure everything is color matched. Sufficient fabric to cover the headliner probably will cost $25 - $30.
Cheapo and easy fix. Put a tarp over the entire interor, and pull off the loose fabric, you will have a crumbly foam surface. Get a small can of latex house paint the right color and paint it with a thick coat. The paint will seal the crumbly surface and don't look that bad in an older truck.
Here is a really cheap and simple fix. Go to your local fabric store and get some "throw-cover" tacks. They are the ones with a twisted metal tack part. You can take a number of them and twist them through the headliner and into the foam-board backing. This is not the most aesthetic fix, but it is very functional. Obviously, you can always do it the right way and replace the entire assy. It will cost about $150-200 depending on source.