We have a sloped driveway also. Our previous vehicle ('99 Tahoe) had a hillholder gear on it that prevented it from slipping forward while in reverse for backing up the driveway. But both of our new hybrids (FEH & HiHy) don't. I'm sure it has something to do with the new Electronically Controlled Variable Transaxles (ECVT).

His and hers hybrids, it's a beautiful thing!