Wow what a wealth of information that I really don't know what to do with.. Thanks everyone for your input. My gut feeling is that I want to use an absolute linear encoder.. This way I don't have to worry about any backlash because the machine will always know exactly where it is because of the abs linear encoder.. I could theoretically also use whatever drive system I wanted to.. be it hydraulics, a single phase AC motor, 3 phase AC, brushless DC, brushed DC, servo, stepper.. whatever... Getting to the point where I know how to implement these things is where I am at right now. I need to figure out what exactly I need to accomplish this..

So I know I would need a linear encoder. Check.
I need some type of driver board to control my device that moves... so do they make a hydraulic controller board or would I have to build this myself? If I used a servo or stepper motor, im sure I could find a driver board to use for this.. but if I used a single phase AC motor or 3 phase AC motor, what driver board would work here?
Then... What would I connect the driver board to? Is there another board that I connect the driver board to before connecting it to the computer??

I know this seems like a very basic question, but that's the point of me starting this thread.. I am starting at the basics and you guys are going way over my head!! please if you would indulge me... I would appreciate if you reeled it back in a bit and explained the very basicness of the solution that I need...

Thanks again for all your input guys. very much appreciated.