SO, have a 72 F-100. 360, SWB, 4x4. Runs really well over the past couple of years with the exception of the infamous rear seal oil leak (the my wife sayls (your truck is crying)).

Anyway, have never had a problem with much of anything over the past few years. Accelerates fine, drives great from 0-70 (although I rarely drive it 70---normally never over 65.

Recently I noticed that I have a vibration that has started.

Truck drives great until I hit about 35-36 mph. At that point it begins a small vibration that gets greater the faster you go up until about 55. At 55 the vibration backs of a bit to about 75% less than the peak vibration. Still a bit of a vibration.

At any point, if I take my foot off the accelerator, the vibration immediately stops. Press the accelerator again, and it starts back up again, right where it left off.

I have done zero mods to it over the past couple of years....no off roading...nothing.

Kind of scratching my head...kind of hard to look under the undercarriage at 40 mph...

Any suggestions?

 

Check the engine and transmission mounts. Use a jack and length of 2x4 to determine whether the metal and rubber interface has separated.

And while under there, do a bumper to bumper visual check and snug up various nuts and bolts.

... perhaps more obvious is did ya lose a wheel weight and thus causing an imbalance?

Road test and post back your results.

 

Sounds like a possible drive line issue.

 

First thing I would do is check your U joints. It is acting like a bad u joint.

 

Driveshaft.
Replace Ujoints.

 

Center support aka "hanger" bearing? Always trash on high mileage trucks.

 

Check everything already mentioned and if you still have vibration, consider new shocks. I've had similar experience on a 2002 3/4 ton and a '96 ranger. After checking/replacing the usual, it turned out to be worn shocks.

 

Tire balance?

 

U-joints...


Took a couple of pics to show, but they will not load properly for some reason. Everything looked fine as I was pulling them apart. Got to the last cap on the second u-joint and basically it was halfway gone. I am saying another 20-30 miles and I would have dropped a drive shaft. $42,00 later and I am cruising likely.

Thanks for confirming my suspicions.

 

U-joints...


Took a couple of pics to show, but they will not load properly for some reason. Everything looked fine as I was pulling them apart. Got to the last cap on the second u-joint and basically it was halfway gone. I am saying another 20-30 miles and I would have dropped a drive shaft. $42,00 later and I am cruising nicely.

Thanks for confirming my suspicions.