I doubt you'll find a typical "RC" servo that will do this job. And even if you do, the price may frighten you off. I see two options for you.
1. Have some kind of power steering system to take the load off any servo.
2. Contact the various servo/radio manufacturers to see what they have to say. Most of them are quite helpful to questions. But not all.
Servo City has what they call servo gear boxes. A large servo with an even larger gear set. Massive torque but travel/rotation time is now measured in seconds. Another possibility from them is a linear servo. You can see both of them here:
Servo City