I've used 2x8 cut into 12-16" lengths, recovery boards, rocks, letting the air out of a tire or three. In that order from most used to least.

If you carry some jackstands you can level and eliminate movement from your suspension, makes your RTT more stable when you move around or in the wind. 17" might not be enough for a 4 runner, so some 2x8" boards might be needed underneath.