My 2002 F150 with 73K miles has had a rear end whine since it was new. The dealer checked it and said everything was in specification, but it's still there. Then I bought mt wife a 2004 Sport Trac, it also has had a rear end whine since it was new. I am sure there are others out there with a similar problem. It looks like this is a Ford design or manufacturing flaw.
Ron

 

What's the big deal with a whine? The rear is still working with no maintenance, right?

 

My '99 F150 did it... my '00 Dakota does it... but my '07 doesn't make a sound.... but I'm sure it will start whining in a couple of months.

It's completely normal of all rear wheel drive vehicles. Infact, it's going to whine more the taller the gears. Like 3.31's for example will be pretty quiet compared to say 4.10's.

You think it's loud in your truck? Try driving a tractor trailer with 8.10's in the rear end. Or better yet, a school bus... I drove a school bus once with an T444e International Engine (basically your standard school bus, front engine, rwd) with a 5 speed Spicer transmission and since it was empty, the echo of the whine would drive you crazy.

Anyways, people are so used to front wheel drive...

 

you need some fancy cheese and crackers, and that whine will go away
but really it should be fine. when i pulled out the 3.31 gears in my setup with over 100.000 miles, it had a whine to it at certan speeds. new setup whith 4.10 gears, whines about the same, but a different speed. wear on the old gears was normal. nothing abnormal but it had that whine.
oh well, just crank up the radio or maybee try some of the synthetic gear oils out there maybee will help out some.

 

my 03 did the same thing and I found out Ford has a TSB out on it and they replaced the ring and pinion for free and it is nice and quiet now. However, my truck was still under warranty at the time.

 

I think id have to disagree with you that all rwd vehicles whine. When my lincoln ls started to whine it was because the diff bearing were shot, got them replaced and quiet as ever. My 82 mustang didnt whine with the original diff or the upgraded one i put in. The 72 mach sitting in the garage doesnt whine either. I can keep going with rwd cars that friends own that don't whine.