'06 XLT Screw 4x4 5.4L with tow package.

When I take off from a stop I feel a brief 1-2 second vibration from somewhere in the rear of the truck. It feels like running over a washboard. At first I thought it was my shovel sliding on the bed liner but I took that out so the bed is empty. Not sure what else to check.

It is very brief and is gone after about 5-10 feet of movement. Shotgun start, turtle start doesn't seem to matter. Straight vs turning doesn't seem to matter.

It has been really nice here lately so I rolled the window down and I can hear absolutely no noise from anywhere. I only feel it.
I haven't towed anything with it yet as I have only had it since July. I am the second owner. I don't know if the previous owner towed anything with, I suspect not since I had to put in the relays for the trailer brakes but that doesn't really mean anything.
I guess I am concerned about something in the rear end. But that may be paranoia.

 

mine does the exact same thing. From what Iv gathered here on the board I am going to change the diff fluid(and make sure to add the friction modifier).

 

Is it a vibration or a thump? I know I have a light thump from my driveshaft due to the slip joint design. Supposedly, greasing the fitting on the driveshaft will fix it for about thirty days. If its a vibration, I'm not sure what it would be.

 

that may do it from what i readed too. i replaced the oil in my differential a couple weeks ago, and used Lucas Oil. 75-140w synthetic. smooth as it should be.

 

I have an 04 and it does the exact same thing. No noise no thump just vibration for about ten feet

 

mine dose it to

 

Same issue here on my '05 Screw. Would love some feed back...just simple fluid change or something more involved.

 

From what I've gathered, it has something to do with the internal clutch pack failing, or sticking together, for the LS rear end. I replaced the fluid in mine when I bought it, and put some friction modifier in it, and after about 10K miles the vibration has only gotten worse. I will probably just end up replacing the clutch pack, or the entire rear end.

 

Mine was doing this the other day. About 2 months ago I had Ford garage replace my axle seals and when the put fluid back in the diff. they apparently didn't put the Friction Modifier in. When I noticed it vibrating I came home and looked at the receipt and there wasn't anything on it about putting the modifier in, just 4 quarts of gear lube. So I go to the Ford garage and get the Friction Modifier, put it in and haven't had any problems since. I've put about 400 miles on since then and it's still smooth so I'm guessing that fixed it.

If you do try this make sure to do a couple figure 8s.