Electronically controlled pneumatic lift, or something similar
A fairly simple pneumatic actuator is the Air Muscle.



It consists of a rubber tube surrounded by a plastic braid. You start out with the muscle stretched out, and when you pressurise it, it contracts with considerable force.

 

The one problem is that they only contract about 30% of the braided length, which means you'll need a muscle about 3 foot long. However, you can have muscles custom made to your required length.

It's possible to use them for precision control, e.g. in a robotic application:



If the weight of the platform is constant, then you simply need to control the pressure in the muscle to control the height of the platform.

If the weight of the platform can vary, then you'll need to control it's position directly. Add a position sensor to the platform, and use a microcontroller running a PID controller to adjust the muscle pressure.