I highly recommend just using a roller to get the paint on and then using a brush to smooth it out. Anything to cut down on the time spent (read wasted) on a freakin stupid vehicle. I mean, we're all gonna die someday. Don't be up there lamenting all that time you spent working on that stupid truck! Gawd. You will be super happy with yourself - cut's way down on masking, time, space, sanding and materials. Way less involved than spraying, you can do it at your leisure. No comparison. And the thing will look great for the time and headaches you had into it - none to little. And you don't even have to sand it really. Weathered old paint will take paint just fine put on with a brush. I don't sand it. If it needs it, scuff it up a little if it makes you feel better.
A 1999 dodge diesel is one thing. Resale value may be a concern. But a well used 1991 dodge diesel is another. You have a work truck. It's work is to take you where you want to go. Not to try to keep it's resale value. That will only go up.
So there are three guys standing around in the wilderness admiring their trucks. First guy says, "I paid 3 grand to have this painted. 5 coats of base, 5 coats of clear". Second guy says, "Sprayed mine myself in the garage. Hung up some plastic, spent a few days sanding and dong the body work and sprayed". Third guy says, "I grabbed a can and a brush and did it in the driveway while listening to the game on Sunday with a cold beer." Moment of silence... then they take off to hit the trails to beat the crap out of and scratch up the things they just spent time painting.
With spraying you end up with a barrier of air between the new paint being sprayed on and the surface. That's one of the reasons surface prep is so important before spraying, got to get that paint to stick. With a brush or a roller you are smashing that paint on there, huge difference. A sprayed house will need paint within 5 years. A house that was done with a roller can last 15 or more. My fathers house hasn't needed paint for 20 years because he took the time to coat it with a roller. Paint sprayed from a gun needs to be thinned typically as well. And spraying paint is akin to projectile vomiting the stuff on there.
For uniform coverage, finish and in certain instances, spraying paint can cut way down on the time it takes to paint something - like a building - spraying is better. There are reasons paint is sprayed obviously, depends on what you want.
Pick some little parts to start with first to get a hang of how it is going to go. Like your rocker panels maybe, or the front valance below the grill that gets rock chipped pretty bad. That will give you an idea of how things are going go so you can tell how confident you will be in appearances, etc.
I have painted a bit of my truck using a brush. For the big parts laid it on with a roller to get it on there fast and wet and then smoothed it out with a brush. Truck looks fine. I get comments all the time. Thumbs up at intersections, people saying things while I get fuel. "Truck looks real clean!! Nice!!" I always have to say.. don't let it fool you, it's just an old truck with 300,000 miles on it. "Really?". Never fails.
Anyways, rambling all over the place. People just spend way too much time on these stupid things.