Check out the link below for a more economical way of doing it. Start at post #14. You can do the whole screw drive for the money you'll have in the lift mechanism above. That being said I have one of the units above and pulled it out of my dump due to lift angle versus weight I could lift for the size of box I have. They're great for shorter boxes but larger trailers and dump boxes not so much unless you use a high torque motor.
http://www.rctruckandconstruction.co...ht=screw+drive
I've not tried linear servos so I cant comment on how they would do but pictures of your set up might help us help you figure it out.