Defrosting and reheating pork shoulder

That’s what I was thinking but don’t have a vacuum sealer. Think it would work simmering it in the heavy duty ziploc freezer bags that I froze it in?

Defrost in your fridge. Reheat in a water bath. Sous vide type treatments are great for heating barbecue or, really, any meal that might tend to dry out or overcook excessively if you microwave it.

Microwaves work by heating the polar molecules in the food (typically water and some oils). Since water does the majority of the heavy lifting, microwaves can max out at about 212F for MOST food that is high in water content. Oil rich foods, though, can go much higher because oils boil at higher temps than water. This is how microwaves can pop corn and cook bacon.

So, if you're trying to reheat something that never brought the food up to 212F during the original cooking, microwaving will tend to want to cook it more than it was originally cooked and dry it out and toughen it. Things like steaks, roast meats, smoked foods, etc. are all on this list. Things like soups, stews, braised dishes, casseroles, etc., all tend to be cooked to at or near 212F during the original cooking, so the microwave can do a great job with them.

A hot water bath never brings the temperature above the temperature it was cooked at, so it does a better job of reheating those kinds of foods that tend to be cooked to a finished temperature well below the boiling point of water. You don't even need the full rig if you don't already have one, you just really need hot water and a way to keep it hot. Depending on you water heater settings, you can just get yourself a cheap styrofoam ice chest and fill it with hot water directly from the tap. If that's not hot enough, you can always heat some water on the stove to get the water in the icechest up to temp and then put the bags of food in and clamp on the lid and just let it do its thing. Periodically you may need to add more water, but it should gently reheat without overcooking.

A microwave CAN reheat it without overcooking and drying, To do that, you're going to need to run it at a low power to avoid it reaching a temperature that will recook the food, stir and rearrange often to avoid hot spots, and it's going to take a while because of the low power. Ovens at their lowest setting will work as well, but their lowest setting can often be higher than the original cooked temperature of the food. Their ability to hold temperature is notoriously crude at the bottom of the range as well. Check often and rearrange the food to make sure you're not getting hotspots, getting temp too high, etc. If you have an electric oven, loosely cover the food with some foil so you don't get the coils cooking the surface of the food directly from radiation.

With all due respect, you are way overthinking this. Pork shoulder is very forgiving and you stored it correctly by adding the drippings to the bag before freezing it. You can defrost it first to speed up the reheating process or leave it frozen if you have time.

In the oven in an aluminum pan with a tight fitting lid until it hits 165°F. Essentially you are steaming it back to temp.