Did some more research. With a 4L engine, the maximum towing capacity for any Ranger is 6,000 pounds. Yours could be less, depending on the transmission, 2WD or 4WD, regular or SuperCab, etc. See the
2009 Owners Manual for specifics. As to actually towing this kind of weight, I highly recommend against it. Big trailers push around smaller trucks, and you can lose control in an instant if the trailer starts to sway, you are going downhill, or you have to swerve due to an accident or blowout.
Also, towing this much would require installation of a heavier duty hitch. Your truck does have a Class II hitch with a rating of 3,500 pounds, as shown in the
Ford 2009 Towing Guide. You would have to upgrade the hitch, if that is possible, to tow the rated capacity. I doubt that even a Class III hitch would have a capacity more than 5,000 pounds. At some point you will reach the limitations of your chassis, and will not gain capacity no matter what hitch you bolt on.
Short answer is that a toybox is not meant to be towed with a compact pickup truck. Even a half-ton pickup is a stretch for all but the super-light toyhaulers, and those will have trouble in mountainous terrain. For anything really liveable, a 3/4 ton or 1-ton PU is a necessity.