Leak from rear right side

That is a lot of liquid. A muffler holding 1/2 gallon of water would, in my opinion be unusual. (they can hold a lot of water, though, depending on the design) Another thought could be coming from the drain in the truck bed. If it had rained or the truck has water in the bed, there are drains that let the water out. Have you checked your brake fluid, it could be a leak from the rear, but not at the quantity you are describing. Could not imagine it would be fuel, it would have a smell.