I don't have specifics because I've never used them, but there are shuttles available from ONT to the DL hotels. What surprised me was how darned expensive they were! They were more expensive from ONT than from other local airports It's actually cheaper to rent a car, depending of course on the number of days, etc.
I have family in the area and always use ONT when I fly to visit them. If I'm going to DL for just a day I rent a car. If I'm going to stay at DL for a few days before/after visiting them I always drive (No Cal to So Cal). If that's not doable, would a family member be willing to drive you to Anaheim?
And yes, you can preschedule an Uber.