Last year we built this super walker with Mini Track Actuators from Firgelli Automations.

http://www.firgelliauto.com/product_info.php?cPath=104&products_id=259

It worked great at first. I could sit and stand anywhere. We even built a second walker with the same mini track actuators. 

But those little actuators just weren't up for the challenge. They kept binding from the leverage on the carriages. One was always faster than the other because there were 2 separate motors with no synchronization. Plus the little motors were too delicate and kept burning out. In the end, 3 out of the 4 actuators failed.