i have a 2004 lincoln navigator. what does it drive like when it needs rear differential oil in it

On a car that new, it should not need rear differential fluid. If you have driven it through high water, or had a leaking seal then maybe it needs service. The lubricant is a synthetic lubricant so it should not need changing yet. But in theory if you did not have enough fluid it would maybe make a slight grinding sound under hard acceleration or hard turning. You'd probably get some sort of vibration in the steering wheel also.

Take it to the dealer and have them look at it. Might make it worse if you keep driving it.

Grinding, scraping noise. It is about time to change it anyway.