I'm no expert but I doubt you will be allowed to re-enter on your J-1 but you will be able to enter the United-States using the Visa Waiver System. Don't forget to fill out the electronic visa waiver at least 72hours before you want to fly to the United States under ESTA.

I had an F-1 visa and when I was done with college but still had my F-1 visa in my passport I always could enter the United States using the I-94W Visa Waiver. I just told the CBP officer that I was done with college and not using the F-1 any longer.