I start with the whole breast while cooking then shred after. I think shredding them and ensuring they're mixed with everything right after cooking helps, but I never eat it right from the crockpot so it could just be reabsorbing the spiced liquid as it cools in the fridge that is doing the trick.

Edit: I also only cook on the "low" setting even though it says 10 hours, I never let it go much longer than 3 or the chicken gets too "well done" just gotta do your typical chicken testing whether with a meat thermometer or pulling it apart to see/feel the texture.