Slow Roasting the Ribs. Another way to precook ribs for the grill is in your oven or slow cooker. Season the ribs ahead of time with a dry rub made from herbs and spices or dried spicy dip mix, then place them on a sheet pan or in your slow cooker. Use the low setting on the slow cooker, or preheat your oven to 250 degrees Fahrenheit. Slow-roast the ribs until they are nearly fork-tender, usually for 2 1/2 to 4 hours depending on the size of the rack. Transfer the ribs to your grill, and finish them over direct heat with your favorite sauce.