I had a similar issue last year. A click when going from reverse to drive quickly. It turned out to be the rear diff. I had a broken tooth on my ring gear and pinion. When was the last time you did a diff service? Jack up the rear end, drain the diffs, and pull the covers. Check the plug for large metal debris. That will be a big clue. Have a buddy turn the wheels or better the drive shaft. Check the ring & pinion and spider gears for chips. Also, if you have the limited slip rear end, it could be worn clutch packs.
It could be something else making the noise. This is what it was for me.
08 Unlimited: Lift, Wheels, Tires, Winch ... and an ever shrinking bank account
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