Is the sap old and dry? Or relatively new and soft?
A few things to try-
First wash the truck then mist the area with quick detailer to provide some lubrication.
Soak a terry cloth towel in hot water (fold it a few times so it's thick and retains heat), place it over the affected area and leave until it cools a bit (not until it's cold). Pour more hot water on the towel to reheat. Repeat as necessary to soften the sap, especially for old dry sap.
Apply Isopropyl alcohol, mineral spirits, 3M general purpose adhesive remover, Megs all purpose cleaner or another paint-safe cleaner product and lightly wipe with a terry cloth towel to clean up the softened sap. Use a terry cloth towel because to has more "bite" than Microfiber, turn to a clean section of the towel often.
Use auto detailing clay and clay lube to try to remove the sap.
I think I remember people have used peanut butter, dab it on the sap and let it sit while the oils get to work... Might want to get a confirmation on this one.
Machine polish to correct any marring caused by the process(s).
Wash and wax often.