I have a 1997 f150 4.6l auto 4x4 ext. cab 4x4 with around 94000 miles on it. In 2-OD(4th) (usually 35-70, maybe -75mph) my truck shakes bad. And when i am in OD and i hit the gas just to gain a little mph but not to shiftout of OD it is the worst! Could it be a harmonic balancer problem? Or what do yall think?

 

Sure the engine could be out of balance.

Have you checked the u-joints? A more likely culprit. Check for play at the pinion and output shaft too.

I'm guessing that you have already checked the wheels for true and tire balance.

Did it just start or did it gradually get worse and worse?

Have you changed anything before it started to happed?

 

it started id say about 750 maybe 1000 miles ago, i had the tires balanced and aligned 3 times, i had new springs and shocks put on, it all started happening after i took it mudding, but i went before and it didnt hurt nething... Where do i check for the stuff you are talking about...the output from the tranny? and the pinion where is that? thanks for the help!

 

pinion-yolk that attaches to drive shaft at the rear end. there is a bearing that can go bad.

Did you check for dried mud on the inside of the wheel lips.

I would also closely check my drive shaft for dents as well as make sure that the balance weight is still there. It will look like a small metal tab welded on the shaft.

A u-joint is still the most likely culprit at both ends of the drive shaft. Because of the spring lift, there is now more pressure on these joints.

Inspect too the rear suspension. If something is loose it can clunk all over.

 

No the new springs and shocks are stock height ...tomm i will go look and see if the weight is still on the driveshaft and i will check all of the u joints...what exactly am i looking for with the u-joints? and just so i can make sure how do i go about checking if the harmonic balancer is ok? thanks

 

the u-joint should have no noticable play front to back or twisting. if you see movement in the joint itself, they need to be replaced. If one is bad, replace them all. they all have the same wear.

You need to check them with the drive train free. If the drive train is loaded up you won't ever find any play. to unload the drive train, set e-brake, chock the tires and with the engine off leave the tranny in neutral.