Yes, it can be removed from below. Just drain the gas, remove the filer neck and hose, Disconnect the pickup line and the fuel gage wire. Remove the three hold down bolts, slide the tank as far to the drivers side as possible and the right side should just clear the upper frame rail and come down. Sometimes this might require some prying with a thin screwdriver or putty knife between the tank flange and the frame rail but it will come down.
Just reverse the procedure to replace the tank.