We use shackles all the time in theatrical rigging, which is what I do for a living. You have to look for ones that actually have a rating or safe working load stamped onto the shackle--they sell cheap ones at some hardware stores that have no actual rating, and aren't tested or certified to be any stronger than a dog-clip.

The reason you want something with a high safe working load is that the dynamic load on them will be considerably greater than the weight of the actual vehicle. The safe working load is generally 5x the failing load (depending on your country's standards) but it's generally not a good idea to exceed the SWL where it can be reasonably calculated.

As a rule your rigging is only as strong as the weakest part of the system, and you really don't want the weakest point to cause a pound of metal to come flying at you as though it was fired from a giant slingshot; which is what you are essentially creating when towing someone/thing. That's the main reason not to use anything with a hook on it, is that there's no positive closure on the fastener, and therefore no guarantee that it will stay where you hook it. A hook on the vehicle is more safe, since if the tow rope becomes unhooked, the hook itself--that is, the piece of metal--is much less likely to come flying at you. Also, stay away from quick-links, if they become distorted under strain they're almost impossible to open/close.

Your best bet, however, is to stick with webbing/strapping (rather than rope) and shackles (rather than hooks) even though they are a little less convenient. It's also important not to store the strap in the elements (for example on your roof rack) as exposure to moisture, and sunlight will cause them to become more brittle over time, and I've seen them snap under load. You also want to be careful about where your soft goods (strap) contacts any metal such as rubbing on the frame (or even using something like an aircraft cable or wire rope) will saw through them.