Strange vibration problem, not your typical symptoms

[blue_goose]

So here's the deal- a rather weak vibration started about 7k ago when going 75mph+ down mtn passes in MT, it did it whether the truck was in drive or neutral(can hit 80 in N). Also when I go about 60mph+ and let off the gas it will vibrate quickly as the rpms fall, it only lasts for about a half second. So I thought maybe U Joints. It didnt get any worse so I drove it back to MN where I could work on it and replaced the U Joints and it recently (last 3k) started a new symptom. I usually drive with the auto 4x4 unplugged at the control module but I had it plugged in for winter driving. I noticed one day as soon as i did that it started vibrating going up hills, only in od, right before it reaches the point where it shifts out of OD. It only will vibrate in OD up hills when the 4x4 module is plugged in, when it is unplugged it just vibrates quick upon releasing the gas or going over 75.

Any ideas? Over the past 7k it has not gotten any worse so I'm just gambling at this point until i figure it out. New fluid in rear diff 3k ago, checking the front and center as soon as I can (this weekend), tranny flush 20k ago. I am hard on it at times (use it like a truck) but take good care of it mechanically. Anyone got any ideas?

 

[happyjack87]

did you check the front diff? i had a similar issue and thats what the local ford dealer said was the issue, not exactly sure what they replaced but thats what they told me.

 

[Got2bneo]

i would check the trans fluid and filter also.

 

[blue_goose]

As soon as it stops snowing and I can get under the truck I will check the front diff. that is about the only thing that would make sense at this point. As far as the tranny and filter, how do I check that? I am not in a place where i could drop the pan and wont be until the end of april. The tranny is still strong, no different then the day I got the truck. Occasionally it may have a hard shift but that is about it. I think I may be able to get 160k out of it.

 

[sk8_jo]

same probleme and the ford dealer dont find anything so if yo find something update the post thank a lot