In a food processor, blend the cream cheese until smooth.
Add Cheddar cheese, Monterey Jack cheese, mayonnaise, pimientos, grated onion, garlic powder, salt, and pepper. Pulse until well combined.
Transfer the mixture to a bowl, cover, and refrigerate for at least 30 minutes or up to 2 days.
Place the chopped pecans in a separate bowl.
Roll the chilled mixture into 1-inch balls.
Coat each ball evenly with the toasted pecans.
Serve the Nutty Pimiento Cheese Balls chilled or at room temperature.