I'd recommend an aluminum racing jack - harbor freight sells one that's very light, which comes in handy, if you're going to be lifting it in / out of the truck. But I think it only lifts to about 18".
While the previous poster recommended Craftsman - I'd recommend against them, based on my experience with one of their 'professional' jacks - the ram failed the 3rd time I used it - and they're only warranted for a year - unlike their hand tools. Swore that I'd purchased my last non-hand tool from them (and they're made in China, too, btw -just like the HF jacks)
I replaced it with a HF 3 ton, which has served me very, very well. But as I look on their website just now, it looks like they've discontinued / replaced it. Plenty of other choices on their site (many similar, it looks like).
http://www.harborfreight.com/automotive-motorcycle/floor-jacks.html?limit=40
The bad thing about floor jacks, for your application (besides their weight) is that they generally have a (lower) handle that doesn't disconnect from the jack easily - not something that transports easily just to have for changing a flat. Because of that, I'd recommend a 'trolley jack' - lighter, smaller, handles that disconnect fully, and sturdy enough to do the job you want. Torin ('big red') would probably do the job you want - you can find them at Pep Boys and other places. I'd recommend them over the Craftsman, at least.
Still, I don't know that I'd even carry a trolley jack full-time in the truck, unless you're moonlighting for AAA or something.
