I have a similar situation and use a Passport (Western Digital external drive - USB) for all of my music files. There are a couple of items that may help: the Passport must be formatted as a FAT drive. Most come formatted as NTFS. Also, the system in my GC natively plays .WMA files, so if you have them stored as .mp3 it has to convert them to play. You can convert your mp3 files to wma using any number of programs, including MS Media Player which comes free with some versions of Windows.
2012 Black Grand Cherokee Overland Summit 4x4
USN Club Hull # WK-13
TEXAS Jeep Club #29