Alarm's won't really do much.
Look into a battery disconnect. Usually around 15 bucks or so.
The only gripe is that you gotta open the hood and remove the knob everytime. The way it works is when you start your 4runner, it needs the AMPS to help power the starter, well the battery disconnect is a disconnecting power switch (still keeps power to radio/clock so no worries) and w/o the knob you can't start vehilce (not enough amps to pass thru). Thus stopping the truck in it's tracks.
Alarms work but if they want your truck, they'll figure out a way to remove the alarm and drive off. Atleast w/ the batt disc they'll have to figure out a way to reconnect it b4 leaving the scene.
But if you want an alarm, try any aftermarket car stereo shop as they can do the work. Heck even CC, Good guys, BB can do it too!
I'd say I personally don't have an alarm though as if they want it, nothing will stop em (damn punks!) - never leave anything valuable behind in the truck (in plain sight).
Good luck!