Did you lose traction while pulling the Liberty? if any of your wheels did it could had triggered an ABS code. Once the truck is shut off and started the ABS proved out ok...

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i know this is six days late but here is the answer to your ?.when you were pulling out you friend the rear tires were spinning and not the front,correct?
the computer sees only this and turns on the light-same as raising the veicle in the air and spinning the wheels.therefore you have 4-WHEEL ABS.if you
had an early model than you would have 2 WHEEL ABS.when you are braking hard all wheels pulsate to prevent wheel lockup.hope this helps.