If your dog hasn't traveled in the bed of a truck before you will want to take steps to introduce her to this new way of travel slowly. Many dogs like traveling in a truck bed, but it can cause anxiety.
Monitor your dog on your first couple of trips to be sure she is handling it ok. Do this even if your dog has ridden in a truck bed without restraint. Make sure she is handling the new harness well and not feeling aggravated.
To ease the process, maybe provide a treat for your dog or your dog's favorite toy. Also, for long rides, be sure your dog has access to water or that you stop often to give him something to drink.