Re: 2007 Jeep Grand Cherokee Laredo Rear Diff Noise
They may have messed up the bearing pre-load when replacing the pinion seal and re-torquing the pinion nut.
They also may have forgot to top up the diff after servicing...though I would hope not!
Take it back, and say that the noise wasn't there before they replaced the pinion seal.
They can easily check if it is a wheel bearing (though from the noise you describe, it doesn't really sound like one) vs. the differential by checking for play in the wheel bearing. Also, if it is a wheel bearing it should alter the noise it makes as you go around curves, since the weight on the bearing shifts (which you state it does not).
Not saying that they for sure screwed up, but they certainly should look at it if the noise occurred immediately after they serviced it.