Best option in my opinion would be to pay the extra $50 an hour and have them remove it before spraying on a new liner.
Anything you use is going to remove it will be abrasive and damage paint. If it takes the paint off, you deal with the bed rusting anyway. If they mess up the paint removing it, they are liable.
If you just wanna go the cheap route, sand blast the old stuff out and roll on a new liner that way.
Oh and a Ranger is not a sweet deal, ever.