Hi,
I have a 2000 F150, S/cab, 5.4l 4x4, automatic. I just had new tires put on, I went up a bit in size, from 235/70/16 to 245/85/16. They are Good Year Wrangler Territory tires, failry aggressive.
Since getting them I have noticed a vibration coming somewhere from the back end ( I don't know if it was there before the tire change, the old ones were in bad shape and made some vibration s of thier own). They were installed professionally and balanced and everything.
Once I hit about 84km/h I can feel it. At first I chalked it up to the aggresive tread on the tires, but then I noticed that it stops if I take my foot off the gas and press the brake, or if I start to coast downhill. So it seems like once the power is removed it stops. So I assume it's not the tires, although the vibration feels similar to an unbalanced tire.
Any ideas?????
Thanks

 

Since its only when power is applied it could be the u-joints or the driveshaft might be out of balance.

 

if you have a l/s diff then it might be time for a fluid change. don't forget the friction modifier.

 

I vote for the u-joints.

 

Thanks for the replies. I'm betting on u joints as well. I forgot to mention that I have noticed a clanky type noise (sorry to be so technical) when I go from drive to reverse. It's not always, but I just remembered that noise.

Are the u-joints really difficult in these trucks???? I do most of my own work, but I'm far from a mechanic.

Thanks.

 

My son's Full Size Bronco was doing the same thing.
When you take off the drive shaft make sure the yoke on the differential isn't loose. We ended up having to replace the ring and pinion because he kept driving it!

 

Get the U-joints that have the grease fittings so you can grease them yourself so they last longer.

 

Thanks again for all the replies. I figured I'd start with the cheapest and easiest. So I got my oil changed and had them do the rear dif at the same time. So new dif oil and friction modifier seems to have taken most, if not all of the vibration out (thanks ChiefFX4!!). I say most because there is still a bit of vibration left, but now it just feels like the new, more aggressive tires and it stays pretty constant when I take the power off. The 'mechanic' at the Pennzoil place that I got it changed at said the u-joints were ok (I know, not the highest end mechanic, but I had him go back and check again and he said he couldn't see any problems with the u-joints).

Thanks again for everyone's help!
Steve.

 

I fail to see how fresh diff oil will stop a vibration going straight down the road. But, whatever works!

 

the l/s in these trucks is pretty finicky. if you don't maintain your rear differential then the clutch packs will start to chatter and cause vibration.

 

No way they will do that going straight.

You have to be turning which requires the clutches to slip. I've experienced this more than once before, the first time in 1968 in my 65 Chevelle 12 bolt posi-traction.