I have the 87 F250 4x4 with a 5.0/auto/AOD. Around 45 MPH, I start to feel a vibration that gets worse as the MPH increases. It gets so bad that the sliding rear window shakes and rattles. When I let off the gas and coast, the vibration goes away and it is smooth as ever. Even if I touch the gas a bit, the vibration/rumbling comes back. I seem to feel it right under the drivers floorpan, but it might be everywhere. Any thoughts? It only happens under load, and the 4WD is disengaged. I thought uit might be the transfer case but does that happen if the 4WD is not on? Manual hubs, 100,000 miles APPROX. PLEASE HELP!!!

 

Check/Replace your driveshaft u-joints. I had EXACTLY the same problem. Grinding/Shaking under ANY load, and nothing if I took my foot off the gas. Good Luck. (get the greasable ones if you have to change em too!)

 

I second that notion. When the U-Joints go out they can make noise and/or vibration. It's a pretty easy/cheap fix...

1988 Ford F250 4x4, 5.0 liter (soon to be Roller 5.0), Mass Air Conversion, Rebuilt AOD tranny, 2.5 Dual exhaust, Headman Shortyheaders, 33" BFG's Mud Terrain, 4.11 gears.



https://www.ford-trucks.com/dcforum/User_files/3c08b4ce6d69304e.jpg

 

Question - with the 4x4 disengaged, is the front driveshaft still turning? Could it be the front u-joints as well?

 

[updated:LAST EDITED ON 04-Dec-01 AT 10:05 PM (EST)]The front drive shaft will not turn if the hubs are locked out and 4x4 disengaged. I think the front drive shaft spins a little when you are moving, but should not spin much, I’m not sure though. Someone will probably correct me if I’m wrong.
Now a story: I just changed my front drive shaft u joints the other day, and they were bad. One of the ends didn’t have and needle bearings in it, and two others were frozen. I did not have any problem with vibrations at all; I just changed them because they were original ford.

 

Madman is right... The drive shaft will spin a little in the front when driving but will not cause the noise you are explaining. Unless of course you have a hub problem which would cause it to spin!