Hello and welcome to just answer Jeep. I am thinking that there is a circuit problem with the seat belt pretensioner circuit in the seat belt buckle. There is a connector on the seat belt pretensioner, and the different weight (lighter) is causing the connector to act up and setting a code/turning on the light. The best way to diagnose this is to connect a scan tool such as a SnapOn scanner to the Jeep and look for an airbag code that will confirm this. If you do see a code related to the seat belt tensioner, then the tensioner/buckle or the connector may need to be replaced. If you do not have access to a scan tool, then check with your local auto parts store. They will usually connect their scan tool for free so you can see what the code is.
Hope this information is helpful, and let me know if you need more assistance with this.