I checked the search and found a lot on vibration's but nothing about a bad vibration when going down hill and off the throttle just coasting. If I give it just a light throttle to put truck under a little load the vibration goes away, if I let off throttle, like free wheeling, the vibration comes back again. Any help would be appreciated Thanks

 

Sounds like a U-joint.

 

x2, I've had the same problem. push up and down on the driveshaft and see if there is any movement, I've replaced the one on the rear diff twice now in 3 yrs, and the front ones had few needles left in em when I got to them

 

I second that vote.....

--Sk

 

SAME. on my rear u-joint both times I felt it in the seat and floor board. at 1900rpms to 2100

 

Make sure there is no torque on the driveshaft when you do this. The best way to do it is to jack up both rear wheels and put it in neutral. If the driveshaft is in a bind when you do this, you probably won't be able to tell which u-joint is bad. Sometimes you have to completely remove the driveshaft to find a bad u-joint. I had to remove mine a couple of months ago to figure out that I had a bad u-joint.

 

Well I went to Auto Zone yesterday and got 2 new joints for the rear drive shaft. I took the shaft out this morning and installed a new front U joint then I pressed out the rear U joint and went to install the NEW U JOINT or so I thought, when I opened the box and out came an OLD AND VERY USED U-JOINT. I guess someone bought this and used the new joint and returned the old one in the box and got thier money back saying it was the wrong one or something. Well A-Z gave me another joint for it but I had to travel 56 miles round trip to exchange it. I would love to get ahold of the JERK that did this and take my gas money out of his HIDE. LESSON #1 ALWAYS OPEN THE BOX BEFORE LEAVING THE STORE.
But anyhow the new joints did the trick and no more vibrations when coasting.>>>> Thanks Bob

 

You can also tell if a u-joint is going bad if there is rust colored dust around the seals.

 

ujoints need a shot of grease every time you service your truck sometimes sooner depending on highway speeds.A dry joint can cause lots damage. Sometimes you dont feel it rightaway.

 

Unfortunately the stockers are solids, w/o a zerk.

When I replaced mine I went w/ Spicer solids again for the strength, but many are happy with the greasables. Just make sure the grease channels are open before you install. Some only get grease to two legs of the joint.