Make the rub:
Whisk together the rub ingredients in a small bowl.
Rub spice mix into roast:
If the roast is tied up with butcher string, untie it. Pat the roast dry with paper towels. Rub the spice mix into the roast all over, reserving any leftover spice mix for later.
Let sit at room temperature at least one hour or chill overnight.
Brown the roast:
Heat olive oil in a frying pan on medium high heat. (If you are using a slow cooker with a removable container that can be used on the stovetop, use that, otherwise use a separate pan.) Place the roast in the pan and brown on all sides.
Put in slow cooker, cook on low:
Place the roast in a slow cooker, and add any reserved spice rub. Cook on the low setting for 6 to 10 hours, until the pork is fall apart tender.
Shred pork, toss with meat juices:
Remove the roast from the slow cooker and place on a cutting board. Cut into large chunks. Then use two forks to pull the meat apart into bite sized shreds.
Return the shreds to the slow cooker and toss to coat with the juice from the roast. (Alternatively you can just shred the roast with two forks in the slow cooker itself.)
Serve with warm tortillas, avocados, shredded cabbage, sliced radishes, cilantro, and a splash of lime juice or salsa.