I recently purchased a 1998 XJ w/ 4.0. The airbag light has been going on and off since I bought it. At some point, the previous owner took the steering wheel apart and replaced either the entire steering column or at least the ignition switch (now has a different key than the door)

Any suggestions on where to start in troubleshooting the airbag light?

 

#1 problem is the clock spring.
Clue # 1 is PO messed with column. They can be fixed.
Check out this thread. Even has pictures!

 

have you retrieved any faults from the airbag module? that would be my first step. double on clock spring. does the horn and cruise control( if equipped) work? try turning the wheel and checking if it causes the horn to stop working. but first thing is first pull the codes and come back and report to us what they are.