Driving a truck that sits level looks better and often handles off-road obstacles with more clearance. Maintenance adds up though, and between parts and the required alignment, you are looking at about $850 for a professional job.
Wait until early spring to pull the trigger. That is when manufacturers usually drop bundle sales on suspension parts, which can shave a chunk off the total price before you even talk to a shop.
Remember that the 0% window is a deadline, not a gift. If you have a balance left over when the 18 months are up, you will start paying interest on whatever remains.