I need to know if I can replace the stock 8 inch sub from a 2001 lincoln navigator with the polk momo mm2084 I can't find any ohmage info anything would be greatly appreciated.

take it out and meter it. but yes it can be replaced

Also check and make sure that sub will even like being in there. Some factory subs are just mounted free air in the back. So check and see if there is an enclosure for the sub.

lincolns have enclosers

and if you can match up the impedance with another sub and swap them out, put some sound deadening in there and you will benefit the most from the change out