'92 Ford Explorer; Vibration under load only between 25-50MPH

[Eraser]

Hello, I am hoping someone can give me some advice on a problem I am having with my 1992 Ford Explorer (4-wheel drive, 150,000miles).

Two weeks ago, a major vibration started to occur. The thing is, it started right after I replaced all 4 ball joints with a friend. A key bit of information here is that we never got it alligned afterward because it didn't seem to be out of allignment. However, I do not believe this vibration issue is related to the ball joints or alignment because of several reasons.

1. The entire car will vibrate only between these speeds: 25MPH and 50MPH. Once I am going faster than 50 or so, the vibration starts to go away. If I am travelling at 65MPH, there is no significant vibration.

2. The entire car will ONLY vibrate between 25 and 50MPH if there is a mild amount of load on the engine (the throttle being applied). That means if I am just barely maintaining speed on a flat smooth surface, there will be little-to-no vibration present.

3. If I am at a speed between 25MPH to 50MPH and my foot is off the throttle, then the car will not vibrate at all (this goes for any speed, actually).

4. The vibration changes slightly in intensity depending on how much gas I am giving the engine.

Thats about all I can think of at the moment.
The car still idles fine.
The car does not seem to have lost any power (it takes off just as fast as before), but I could be wrong because when the car is vibrating, it messes with my senses.
I changed the spark plugs and wires two days ago because I had hoped it could help, but it didn't, and I had a feeling it wouldn't. I also cleaned and re-oiled the air filter element.
I have plans next week to replace the fuel filter and the 2 U-Joints on the driveshaft that leads to the rear axle. Also will probably take a look at the rear brakes.

This problem is very annoying... I would greatly appreciate any assistance anyone can offer!

 

[Glacier991]

It would seem odd in terms of any connection to your ball joints, but you describe a failing U joint almost exactly. Without much torque it is still strong enough to maintain it's position. Load it up and it moves...enough to introduce a drive line inbalance and vibration. Get a KLUNK on putting it in gear? Try putting it reverse and then soon as it goes into reverse put it in drive... without any revving... do you get a "klunk"?

That's there I would start in my search. These things can be puzzling, but in most cases they are U joint, tires, alignment or in rare cases something entirely different... often shot Radius Arm bushings can play into the problem as well, or bearings, or... or ... or...

 

[Eraser]

DUDE!

I think it is the U-JOINT!
I get that klunk you are talking about when switching between reverse and drive. It started at about the same time too! It was buggin the heck out of me as well!

oh man that would be awesome if its that simple. I will know for sure next week when I get it fixed with my friend (he's a service manager at firestone).

What's funny is I suggested replacing my u-joints before the problem started... its like I have a psychic connection with my truck or something hehe.

Time will tell. Thanks for the help!

 

[Glacier991]

That means that you DO have some U joint play... whether that is *IT* may yet be another matter... but the symptoms point there pretty clearly... even if you replace the joints or a joint... it MAY not be the problem, but will be money well spent. Work thru that first and see if the vibration goes away. I'll keep my fingers crossed.

Btw I have 210,000 on my 92, and have replaced the U joints.