On my WJ, it was a bad airbag control module (ACM). I just replaced it with a used one from an '04 from eBay, and the issue is gone. The module is under the center console.
'06 XK Limited, 5.7L, QD II, Black, 163K
'02 WJ Overland, 4.7L HO, QD, Inferno Red, 126K