The vehicle should be safety inspected every oil change, that's my opinion.
The brakes should be inspected every oil change and replaced when necessary.
Assuming you mean "replace" the brakes, the brakes should be inspected for condition every time the vehicle is serviced and repairs, if any needed, should be done at that time.
Have brakes, front suspension and steering inspected now.
If the brakes on a 2003 Toyota 4Runner are locking up there is a problem with the calipers. The units should be inspected for wear, to avoid any serious injury.
Have the brakes inspected by a trusted garage every oil change or at least once a year.
It would depend on the driver's habits and the vehicle. I would recommend they be inspected by a mechanic every 10K or once a year.
Warped brake rotor(s). Brake pads and calipers should also be inspected.
You start by adjusting the rear brakes after you have inspected them.
There can be. Only way to know is have your brakes and rotors inspected.
One wheel of the brakes may be "grabbing" because of cracked brake linings or broken hardware. Have the brakes safety inspected soon!
The most likely cause is a warped rotor. It can also be other things so have you brakes and suspension inspected.
Weather doesn't matter. Have the brakes inspected asap before something happens.