When I HotShotted we pulled all of our vehicles from centraldispatch.com. All the vehicles came through brokers so you may be able to check broker ratings and find someone on there. Everything is legit as far as insurance and legalities throught that site.
The cheapest way may be Uship.com. You will have to do your own legwork as far as checking people's ratings and insurance but it will probably save you a couple bucks.
Both these options are for putting your truck on a half full trailer as filler and therefore cheaper than a special trip.
If as noted above you can let it be filler on another trailer it helps the price a ton (what DAS does). You generally won't have absolute dates and it may be really slow as it moves across the country as filler on a couple different trailers. If you need absolute pickup and delivery dates it can get expensive quick.
U-ship.com is a good site... has many options to get the best price... I had a car shipped from Nashville TN to Dallas TX for $400 and it took 2 days. I was very satisfied.