On the most popular rivers there are usually shuttle services that do nothing but shuttle vehicles. There are two on the Yellowstone in the Livingston area, Hooters and B&G's. Since the river is very heavily floated and has very convenient access points, it's probably a profitable business. I think the way they do a lot of it is to use a big van to drive a group of drivers to the uppermost put-in, drive those vehicles to their take-out, which is the put-in for the next group of vehicles, drive those to the next access, and all the way down to the lower end. Most of the guides seem to put in and take out at about the same times. I can speak from experience that they gripe a little when you put in a lot later in the day and mess up their system. Mostly you call them, tell them where and when you're putting in and taking out and when you'll want your vehicle at the take out, give them your license plate number and where you'll leave the key, and where you'll leave the cash in the vehicle. This year they are charging from $30-45, depending upon how far away your float is from Livingston.
There was a guy last year that set up at the take-out on the lower Madison with a big van and drove people back to the put-in to pick up their cars all afternoon. Since the lower Madison is Bozeman's and Montana State's party river, it's crowded enough that he was kept really busy.
Some areas, like in the Ozarks, have canoe liveries on the rivers that will shuttle private vehicles, but this is getting rarer because the insurance companies don't like them driving somebody else's vehicle. So many are now loading your canoe on their vehicle and driving you to the put-in, or if you're putting in at their place, driving their vehicle to the take-out to pick you up.
The sweetest way to do it if you are using whatever kind of boats that you can carry two to a vehicle (canoes, kayaks, pontoon boats on larger trailers, etc.) is to do piggyback floats. Two to four anglers in two boats, all in one vehicle, on a river with two back to back float stretches of similar length. You drive to the uppermost put-in, one boat and guys put in there, the other guys then drive the vehicle to the first guys' take-out, which is their put-in. They put in, and the vehicle is there for the first guys when they arrive at their take-out. Then they load up and drive it down to the second guys' take-out and pick them up. It's a do-it-yourself shuttle that saves a lot on gas.
