This is a very common problem with E4OD. There is no risk of damage, but start saving up for a new converter. Mine went out at 115k, about average, according to the trans shop. You will suddenly get VERY hard shifting when it finally fails. This is from the computer maxing out the hydraulic pressure to get the clutch to hook up. You can still drive it after that, but you will want to get it fixed.

I had the whole transmission rebuilt, but it was expensive, $1800. Without the slippage, my highway gas mileage has improved by 2-3 mpg.