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Shoulder is just one tough cut of pork and a lean one doesn't help things any. The good news is that a shoulder has a lot of tough connective tissue that melts at the higher temps, keeping the meat juicy.
3 days in the fridge with just a surface rub, especially a salty one on a leaner cut of meat, can pull a lot of essential liquid out of the meat. Try cutting that to 24 hours or not at all, just putting the rub on before it goes on the smoker.
Another suggestion would be to inject your lean shoulders with brine then brine it for 18-24 hours in the fridge. A brine puts liquid and flavor into the meat because the liquid in the meat and the liquid in the brine change places down to the cellular level. Now, most of that liquid will evap out at lower temps on the smoker at the stalls, but the brine provides deep flavor and a better start to your smoke as the your leaner pork shoulder climbs through the physical and chemical changes on its way to your 205F target temp.
Inject, brine, wash, dry, then rub just prior to putting on the smoker. Can't hurt to give it a try. I have two leaner looking, injected boneless pork butts brining as I type in fresh pressed apple juice, spices, and a little apple cider vinegar. You could skip the pink salt if you don't have any.
Apple Juice Pork Brine
Makes enough for up to 12 lb of meat.
1/2 tsp Prague #1 Pink Salt
1/2 cup kosher salt
1 Tbs onion powder
1 Tbs garlic powder
1 tsp black pepper
1 cup dark brown sugar
1 quart Simply Apple fresh pressed apple juice
1/4 cup Apple Cider Vinegar
1 quarts ice
1 quart water
1. Using large pot or Dutch oven to mix all the brine ingredients except the ice and the last quart of water. Stir over low heat to mix and dissolve while stirring. Turn off heat as soon as all the ingredients are mixed. Add the ice. The brine should be cold before adding the pork.
2. Rinse the fresh or thawed butts under cold water then place in the brine. Inject the meat while sitting in the brine. Pour the remaining brine and last quart of water over the meat, seal or cover and store for 12 to 24 hours in the refrigerator or a cooler packed with ice jugs. Brining for more than 24 hours is not recommended. This process will produce a tender, juicy final product because the salt in the brine changes the protein structure of the meat.
3. After brining, drain and discard the brine. Rinse the pork again then prep them for smoking, baking, or BBQ'ing.