1999 ford ranger 4.0 v6 4x4 with 217,000 miles. Ok here goes. I turn on truck, runs great, i drive, runs great, then suddenly for no apparent reason there's a BOOM and it feels like something dropped and the truck starts vibrating and sounds/smells like the tire is hitting something...so i slow down as fast as i can. when i drive again EVERYTHING is back to normal, THEN BAM! again! like something dropped, lotsa vibration. truck does NOT lose any performance, steering does NOT act up. NOTHING changes cept i can smell burning rubber like the tire is hitting something. And it WILL NOT stop no matter what speed i'm going no matter if i slow down or speed up it WILL NOT stop UNLESS i bring the truck to a COMPLETE stop. then, voila, problem is gone! until it decides to come back. problem happens at various speeds, but usually above 45mph, but then as i slow it DOES NOT go away unless i come to a complete stop. then it starts over. i have no freaking idea what this is about. tires are a little old, but wouldn't the problem be there always if it was a tire? engine mounts are problem old, but again wouldn't the problem stay all the time? and the tire burning smell, i cannot find anything to indicate it's touching the tires. entire front suspension has been replaced and looks tidy. engine mount sounds most likely but why the rubber burn smell? and why does problem persist no matter what unless i stop? it makes absolutely no sense to me. i've looked up all kindsa vibration problems, and most make sense. it happens in neutral, or at a certain speed, or gets worse as speed increases. my problem exhibits NONE of those! it comes on fairly random and then persists until the truck is stopped, no exceptions. WTF can make this happen?

 

I had almost the same thing happen.
I would get a bang or loud thunk and the truck still rolled forward until I slowed down and thing started to jam up.
Stopped and went forward again all OK until the thunk again.
It turned out to be my rear brakes, the shoe supports broke and things got tight.
Had the shoes, all of the little parts and the drums replaced, all is good now.

Not that this is your problem but something to look at.

 

hey scrambler. thanks for the reply! it certainly feels like it's coming from the front, but i can definitely check the rear brakes. the whole damn cab vibrates. i can feel it in he pedals. i can feel it very slightly in the steering wheel. but there is certainly a loud THUNK before the thing happens. and it will NOT go away until i stop.thanks for the suggestion!

 

brake drum issues would be my guess as well, most likely a broken spring.

 

You may want to look into your 4wheel drive components. Sounds to me like it is engaging on it's own. Possibly a vacuum leak.

 

OMG POWER RANGER U ARE A FREAKING GENIUS!!!!! it was my front driver's side wheel trying to engage on it's own! the freakin plastic guide for the gear was all broken and melted (that's the smell i got) for whatever reason when i would hit a bump with left tire (big or small) it was drop down and try to engage..which is basically impossibly while the truck was in motion, therefore causing my crazy vibration sound and why it would go away after i stopped...thank you so much you saved me a HUGE headache and TONS of money :)

 

Nate wins.

Glad you found the issue.

 

No problem man. Reason i thought you should check is because my f150 was doing this because of the lovely IWE/solenoid/vacuum leak problems that tons of them have. Glad i could help.