Legal? Absolutely.
Easy? Nope.
Safest option? Nope.
The law doesn't care too much what vehicle you put your child in as long as he's porperly restrained. However, Toyota Tacomas are notoriously difficult for installing car seats correctly. And, in general, pickup trucks are not the safest transportation for kids. The cabs are just too small.
If there's any other options, you might try them. The Toyota Tacomas are notorious for being difficult to install car seats, forward and rear facing. Especially the extended cab/access cab versions. People have a little easier time with the 4 door one.
Check out this thread on a car safety forum discussing the problems others have had, and techs chiming in on solutions:
http://www.car-seat.org/showthread.php?t=26992
http://www.car-seat.org/showthread.php?t=15652
http://www.car-seat.org/showthread.php?t=2950
http://www.car-seat.org/showthread.php?t=1838
http://www.car-seat.org/showthread.php?t=996
It may only work for you to shut the airbag on the passenger side off and put the seat up front, if you absolutely have to use the vehicle to transport a child. But make sure the airbag is off! Vehicles after 2001 should have a button or sensor to shut it off. Older vehicles you will have to take to the dealer or a mechanic to get the airbag shut off.
Apparently neither rear or forward facing car seats work well in this vehicle, so its definitely not the greatest transportation for a child, something to consider. Actually, pickup trucks in general should only be used to transport children as a last resort - they just are nowhere near as safe as the backseat of a car, van, or SUV.