I'd see about fabbing it to the spare tire carrier and having the tire out farther, but then this adds a bigger torque arm to the hingers.
What I had in mind was to take a 'L' bracket and secure it to the tailgate closest to the hinges as possible. This would be the base for the base of the lift. Then mount another 'U' bracket up higher in the tailgate to provide vertical support to the lift.
The lower bracket would be shaped like a step. The base secures the base of the lift. The mounts are secured to the tailgate with a cross-bracket on the inside. The sheetmetal won't be strong enough to hold the weight. However, if the inside cross bracket was secured to some structural crossmember of the door, then through the tailgate to the mount, it should be strong enough for that. Then the same inside tailgate support for the upper bracket. Again, keep as close to hinges as possible.
I wouldn't suggest that you place the hi-lift up front unless it's high. They do bend.