I would have a tranny shop familiar with the 4L30E pull the pan and check the tranny out ASAP. I'm not sure how the TCM determines the slippage, but it's ususally not a good sign with the 4L30E. Could be a weak solenoid, band, or maybe accumulator spring? Hopefully you can catch it in time & resolve the issue. These trannys dont hold up well at all if they are not maintained. ATF changes every 25Kmi, or sooner if you want to keep it forever. G/luck
Joel