1) check u-joints
2) if you can feel a bit of a thud along with the noise then grease the driveshaft slip yoke

 

sounds like a transmission mount to me... if its long bed...does it have a carrier bearing?

 

Well, over a year later and here's what's up. I have driven this truck about 8k miles since I posted this. There was a fairly long stretch where I couldn't duplicate or notice the issue at all. When I checked u-joints back in NOV 2011, I couldn't find any play or movement at all. A few months ago I started getting these vibrations coupled with your classic bad u joint clank at certain invariable speeds, some times upon accelleration and sometimes upon decelleration. The clank is more of a feeling. IF there is a noise it must be very suttle....like I said, more of a feeling. In the past few weeks I started driving the truck again after it sat for about 3-4 weeks. Now, A MAJOR vibrating shake occurs especially at 30 and 50 mph. I've ruled the tires out. With truck jacked up and rear tires and wheels off, i can duplicate so it's not tires or wheels. I was convinced then AND now that this is not a transmission problem. I've learned (I think) a lot today as far as I'm not the only F-150 w/ fairly low mileage having this problem. Also that FORD used "unservicable" u-joints which are "spiked", not clipped. Have found a lot of contradicting info plus a good bit of some guys being "creative" and getting around this supposed need to replace the entire drive shaft and joints. Let me hear from you, fella's. I gotta figure this out quick. Thanks a lot and hope the new year is going good for everybody.