The friend I bought my '13 JKU from said it was grinding and/or hanging up in 4th right after he bought it. Took it to the dealer, they "fixed" it, and now it will pop out of 4th and grind if I don't keep pressure on the shifter while releasing the clutch.
It also rattles in 4th, and the only thing that will make it stop is pressing down (toward the floor) on the shifter.
I'm taking the Jeep in to the dealer for service this week (trying to maintain my 100k drivetrain warranty...), I'll have them take a look at it and will post up what they say.
2013 Billet Silver JKU Sport S, 6-Speed, TF Leveling Kit, Spidertrax 1.5" Spacers, 285/70R17 Firestone Destination M/Ts on OEM Sport wheels.