Check the antenna on the roof. It unscrews and may have come loose, reducing reception.
I wasn't sure if the antenna mast was part of the XM receiver but it certainly won't hurt to check this connection.You can try that, but I think that's for AM/FM. XM and GPS would be part of the base. It could be the antenna module or the radio module. The dealer should have a troubleshooting tree that would guide them though that.