I would think it shouldn't take much more than an hour or 2 at the most. Most of the time cutting the bolt is the easiest and fastest way to remove the old. So 2 Shackles and 2 bolts/nuts. If they charge you more than $4 or 5 hundred total I would say no.
It is something you could do on your own. It is fairly easy. Support the frame with a jack, use a grinder with a cutting wheel than install new. If you go that route I would get some POR15 or similar rust paint to slow down the rusting since you don't plan on keeping the truck much longer.