How to drive a linear actuator in both directions
Use an H-bridge. It is cheap and straightforward to build. 

http://www.bristolwatch.com/k150/port4.htm

I use this for high current switching: http://www.bristolwatch.com/k150/pics1/mosfet_hb2.png

These N- and P-channel MOSFETs have a fairly low V_DS(ON) resistance, and don't run very hot at 2A. P = (I^2) x R = 4 x 0.8 = 3.2W for the IRF9630, less for the IRF630. 

I don't personally like BJT transistors in H-bridges except to control the MOSFETs, but perhaps there are scenarios where they're better. If so, others may chime in with their cluebats.

Hope it helps.

Edit: the linked schematic is not for PWM (I used it for 12V actuators as well). Cost was low, &lt;5USD