2003 Ford Expedition - Two Part thread

[Ghost of Spart]

Or at least I'm guessing it's two part. They might be related.

Question #1 - Where I live, we've had a pretty brutally cold winter. I've noticed on the most punishing mornings(20 degrees and lower for example), my Expedition has an odd screeching sound at it accelerates up to speed. If you open the door up and hang out, just idling along it sounds pretty pronounced. As you accelerate up, it gets faster, and then just disappears. I'm wondering what it is. I crawled under it last night and checked the universals, and they seemed find. But I got to thinking about it this morning, and I didn't have the vehicle in neutral, it was in gear. Don't you need to be in neutral to check a U-joint? It's got me stumped. I can't tell if it's coming from a specific wheel, like pad is dragging. What sounds possible?

Question #2 - Alright, I have the infamous growling differential problem in the rear. Low speed turns or under load turns cause me to get the rumble strip sound in the rear of the Expy. Everything I've read says it's the clutch packs in the rear. Here's my question...is this a problem that is going to get worse, and what is the potential negative outcome? I'm probably going to go this week and get the gear oil changed and the friction modifier put into it. I've never messed with a rear end, so I'll be taking it to my mechanic. He's really proven that he's trustworthy, so I don't expect to get lit up too badly. Is changing that oil out a big undertaking? Hard to do? But bigger picture, what is the repercussion from just letting this go? That's my biggest concern with now. It's not an all the time thing, like everyone has mentioned. Just those low speed turns in parking lots and the like.

Any comments or help would be greatly appreciated.