Hi Sam,
According to PIC-Design, a ballscrew 3/4" in dia with a fixed support at one end (2 bearings 1.5 times the screw dia apart) and a simple support (one bearing) 53" from the fixed support will have a critical speed of 1200+ rpm. With a .2" lead, this would give you 240ipm before oscillation starts. With a .5" lead, 600ipm!! You could directly couple a servo and be right in the max torque range, but if you coupled the motor to screw using a timing belt, you would have something to give way before destroying your machine in a crash. At least I think it would work, but I don't want to test that theory, do we have a volunteer?
Please don't use my figures without double checking them, I'm not an engineer, I just ran your numbers through the PIC-Design critical shaft speed calculator at
http://www.pic-design.com
Good luck finding what you need,
Steve