I saw this link on you tube:
https://www.youtube.com/watch?v=GjfsPTJx_DY
It's not necessarily this HS-55 motor that intrigued me, but the attaching piece to the servo. While it's not truly a linear actuator it could serve that purpose with a little math. The thing I find most attractive about using servo's is the ease of setpoint operation use. The question then remains what servo's of what high torque are allowed.
I'd sure love to use something like this SDX-901 if it wasn't so expensive
https://www.youtube.com/watch?v=YDFMhyA4rHw
the HS-55 is only 18 oz-in (0.127 nm, 0.09 lb-ft)
while this SDX-901 is a whopping 402.7 oz-in (2.84 nm, 2.09 lb-ft)