['01 F-150, 5.4L w/3.73 LS rear end. 207,xxx miles!]

In the last month, I've noticed that from a stop when accelerating AND turning, I get a grinding accompanied by a shake. Some research on the forums here has led me to believe that instead of a problem with the spider gears like was suggested to me before, its the clutch packs not slipping properly and that the problem is easily solved with some friction modifier. I'm hesitant to try because...

... I'm pretty sure there is a bearing going bad somewhere behind the drivers' seat. I can't tell if its coming from one side, but its obviously from the rear of the truck. I have someone opening up the diff and checking wheel and diff bearings and replace as necessary this weekend. He is the same person that suggested that the grinding was a problem with the spider gears and suggested replacing them as well.

First: I know there's plenty of people with similar issues here, how many of you have had to replace spider gears as a result of some grinding?
Second: I know this has some miles. But I don't have that much money to just be spending hundreds on bearings and installation. If he gets into it and finds that the bearings (either wheel or diff) aren't bad, should I still have him change them?

I know you guys have helped me before, and any help now would be awesome. Also any suggestions, tips, comments...

 

I would replace the lube and add friction modifier before I allowed anybody to tear into it. My 2000 did the same thing at about 80k miles. I changed the fluid and added one bottle of modifier. Still had the same problem. Then I found out that there was a TSB on that model saying if the problem occurred to replace the clutch packs and reduce the shims by 0.010". I talked to the parts guy and he said most people just add a second bottle of friction modifier. So, that is what I did and never had the problem again.

 

How long did it take after you added the second bottle for you to notice the sound going away? And when you say 'the same thing' are you referring to the grinding?

I just took the diff cover off to get a good look at the gears (concerning the grinding noise) and they looked incredibly good for as many miles as it has. Not perfect, but good.

To check some bearings, I tried shaking the driveshaft, no play (pinion bearings OK) tried prying spider gears up, no movement (can't remember the name, but this bearing is OK too )

I put in two bottles of the Mobil 1 oil in the rear end as well as a bottle of the friction modifier. I took it for about a mile drive... doesn't seem any better.

 

Add another bottle, then drive it slowly in figure 8's for about 15 minutes.

 

The one bottle seems to have helped significantly with the grinding noise. Unfortunately, the hard part is yet to come, as I still have absolutely no idea where the growling is coming from. I'm quite sure its from the rear, but like i said, I'm always in the drivers' seat to determining what side (if any) the noise is coming from is next to impossible.

 

The noise went away almost immediately with the second bottle. With 207k miles though you may have other problems.

I once had a bad front pinion bearing and I thought it was roaring tires. I actually had the tires replaced and the noise was still there. I pulled the wheels off and noticed I had oil leaking from the right side axle seal and the pinion seal. I then disassembled the rear and found the bad front pinion bearing. I replaced the one bearing and reassembled it myself, the second time I had worked on a diff. It had just a little sing in the gears afterwards but wasn't bad. I've ridden in new cars that had as much noise in the rear gears.

 

Being that the fluid change didn't help... I spent the $300 on all new bearings. Have a friend tearing into it tomorrow night. He said if something doesn't need replacing, he won't replace it.... still gonna cost me an arm and a leg.