I have a Navigator with the THX system that I replaced the factory sub with a Polk MM 8" powered by a JBL amp. The factory sub is DVC and IIRC was 2ohms/50watts per coil. It was stamped on the back of the factory sub. If you have other than the THX it may be different. If you plan on using the factory amp I would pull the sub to be sure first. There will most likely be a bolt through the back of the sub box in addition to the screws in the front to get it out. You will almost definitely need a space ring to fit a new sub. If using the factory amp I would recommend going for the most efficient sub you can get.
I also usually do anyway but in this case sound deadening made a huge difference. Just adding the sub and more power made for lots of vibrations and sounded worse than stock. Adding a less than one sheet of vibration dampener and closed cell foam around the edge of the sub to create a seal between it and the grill on the back panel it was night and day.
Even if you add an amp don't expect it to thump. It's an 8 in a large truck. It can only create so much air pressure. I am happy with the results as it is louder, clearer and there is more bass, and I can turn the whole system up louder now without distortion. That being said I do plan on adding a 12" in a custom box I can remove when I need the space and plug the 8" back in.
Lastly new speakers need to break in. Don't crank it until it has at least a few hours on it and don't be surmised if it is a week or 2 or even more before you can really hear it shine. It was prob a month before I really was happy with the upgrade.
