From my own experience and I'm not a pro, sandblasting a car body is difficult to say the least, and you could be headed for trouble if you don't know what you're doing. I used to work for a sandblasting company and the boss refused to do any cars because of the problems associated with it. Only one guy in the crew blasted cars and he taught me whatever I know about it. For the most part, you don't want to sandblast the whole thing unless it's that bad. It's a 96 so I'll assume it's not that bad. If you want to blast to remove the paint, try a chemical stripper instead. I wouldn't recommend blasting the fiberglass unless you have osmosis and I don't think you do unless you've been using the truck as a U-boat.
We are talking about the box, so if you're blasting rust, and there's a whole lot of it, you should take it off the frame. You may want to rent a compressor (like 185 cfm) and a sandblasting pot. Make sure you also rent a hood with an external air supply and lots of lenses. I use shot glass for media although my next car will be blasted with coal dust or another media that my brother is using (I don't know what it is, but it's black), I also might try walnut shells they worked great on some aluminum rims I had. You want to start with low pressure (80 psi or so depending on your nozzle) and keep the nozzle moving, heat from blasting can warp the panels or you could blow right through it. Adjust pressure as you see fit, but remember the more pressure, the more heat generated, the more warpage. Like I said, this is the kind of sandblasting you have to know what you are doing or you'll wreck something, spot blasting is probably what would suit your situation better.
If you're blasting a few spots, (the box can stay on the frame)then a shop compressor is usually enough (5 hp + 60 gal tank or better). The handheld blasters are OK, but (depending on the amount of rust) you need a lot of tips and you'll be spending a lot of time on it. The little pots are quicker, but you need a bigger compressor. You can spot blast wherever you have rust and not be too worried about blasting through good steel or warping. Again shot glass is a good media, but I still recommend a hood, lenses, and an external air supply.
The blasting media will get everywhere. I'm not kidding, everywhere. If you choose to use the big compressor, have a lot of room (like a field) or a blasting booth. Keep in mind that any openings in the truck will get media in it, that's why I said to take the box off the frame. The other thing is to prime right away to stop further rusting from happening.