I'm just curious, but how would this help, and also, have you seen this issue/fix before? Thanks in advance if it works. It's over 100,000 miles so it's time for a rear differential service.

I had an 04 fx4 with about 105k and it developed a mild shake around 20mph. That's what the local ford shop suggested so i had them put a bottle in mine. It did seem better but not all the way gone and unfortunately i rolled the truck not long after so i never found out if that was it. They said the limited slip can do that. It's worth a shot and a cheap fix if it works. Good luck.

Well, I serviced the rear differential today. I used Royal Purple, which said it had a friction modifier included, so I haven't added any friction modifier. I torqued all the pumpkin bolts to 18 Ft/Pounds, with Permetex. (Whichever one was for the gear oil.) I used Permetex thread sealer on the fill screw. (Also, I know a bunch of guys only use Motorcraft oil in these trucks, but they didn't have any Motorcraft gear oil in stock. So please try and give me constructive comments regarding this.)

I'm gonna give it a drive tomorrow, and see if anything is different. I'll be shocked if this fixes anything, but at least it rules one more thing out. Plus, it probably needed to be done anyway.

Is it a high frequency shake like at driveshaft speed or a lower one like at tire speed?

FYI..friction modifier is only gonna help with differential clutch chatter while turning

I realized this was the case about the additive, after talking to the guy at Oreily's.

As far as everything else, I went back, and torqued the cover bolts to 33 ft'lbs. I also added a limited slip additive, as the Royal Purple didn't have it.

I don't know if this helped anything. I felt like it might be riding a little smoother at 52, but I can't tell if I'm just hoping it is or not. I'm gonna drive around for a week, and really see if I can tell the difference, especially up at 72 where the problem also happens.

Honestly, I think it's related to the tire speed. You know, saying it out loud, it almost feels like there is the same speed of shake at 52 mph and 72 mph. What the heck would that mean? Also, I'm not sure if you read this entire thread, but I did replace the driveshaft and rebuilt the Ujoints on that driveshaft.

Bump here.

It's been a long time, and I still can't come up with what could be causing all this. Any other ideas?

Mine is wind buffeting over 55 Mph... been there since new. The dealer felt it too. They replaced the drive shaft, tires, wheels, back brakes, etc. It never went away... of course the dealer said it was gone. 165,000 miles later... still there. I chalk my vibration up to wind buffeting. I got lucky with some Cooper ATP tires and a good road force balance this set and was happily surprised to find it was less apparent.... go figure! Removing the factory rear mud flaps decreased it some too. Good luck!

I appreciate you chiming in, but I'm sure this isn't wind. When I hit the brakes, I feel like the shake is more prominent. Also, I'm about to do a timing job on this truck, because the check engine light just came on last week, I pulled a code, and it was related to the timing, P0016 I believe. Could be unrelated, but I feel like since the check engine light has come on, the shaking has gotten worse? Maybe I'm going nuts though at this point.