https://egov.uscis.gov/casestatus/mycasestatus.do

enter LIN number and it will post (after approval) a tracking number.

Should say:

On March 12, 2015, the Post Office picked up mail containing your new card for Receipt Number LINxxxxxxxx26. We mailed your card to the address you gave us. The tracking number assigned is [lots of numbers go here]. You can use your tracking number at www.USPS.com in the Quick Tools Tracking section. If you did not receive the card, please go to www.uscis.gov/e-request to request that we resend the card to you. If you move, go to www.uscis.gov/addresschange to give us your new mailing address.