2001 Excursion 4x4, 185,000 miles original automatic transmission and 7.3 engine. For a little while now, I would say the last year or so, I have noticed driving down the road or highway in fourth gear only, between 50 and 60 miles an hour there is a noticeable shudder coming from the drivetrain that you can feel all the way through to the steering wheel. this was very slight at first, thought maybe i just needed a wheel balanced or alighnment, but after a new set of tires and alighnment the problem is still there. Have checked u-joints and driveshaft for balance and are all fine.
Noting that this is only happening in fourth gear between 50 and 60 mph and the shuddering is getting more violant leads me to think maybe something with the transmission???? This truck is not a daily driver and may sit 4 to 5 days at a time before being driven, i guess thats how i can tell the problem is getting worse! Plan on taking it to a shop this week and getting it checked out, i guess i am just looking for a place to start. I could be completely wrong thinking it's the trans. Any input would be greatly appreciated, thanks in advance!

 

Have you greased the slip joint on the driveshaft?If it is dry it will cause shudderring.Cheap/easy fix.Are the strap bolts at all u-joints tight?
You said you checked the u-joints,a small amount of slack or a dry joint may cause binding.

Slip joint regreasing

 

Is it lifted at all?

 

When you refer to slip joint are you talking about the yoke at the rear of the trans or the front driveshaft? thanks for the response.

 

The slip yolk is the one in the front of the driveshaft, the back side of the carrier bearing. It has to move as the real axle moves when it hits bumps and potholes, and when you load it up.

 

The 3rd and 4th pictures in this link Slip joint regreasing show what you are after, it is an easy fix, just be sure to chock your wheels before starting.

 

Are you sure all 8 cylinders are working? These engines will run fine on 7 cylinders and shake in top gear when accelerating from 45-60 mph and then smooth out after that. If the trans downshifts to third, the shudder will go away. A weak injector will give similar symptoms as well.
How is your mileage? Any SES light by any chance?

 

To JMathews the answer to your question is "yes" the truck is lifted. Tough Country 3.5 Kit, springs in front, blocks in rear, dual shocks up front, dual stabalizers. Rear driveshaft was lengthened and balanced.

 

To GChavez, Have noticed a slight drop in economy! What is an SES light??

 

Service Engine Soon.

Does your shudder go away when you let off the throttle?

 

GChavez, yes shudder goes away when off the throttle. Shudder is more noticeable under load, taking a grade i.e.

 

Mine shudders when the rpm's are low and it goes into OD about 45mph. The cause of my shudder is I have one or two bad cylinders with low compression.

 

Your symptoms point to a bad injector. A buzz test might give a clue as to which one. Is there anyone in your area who has AE or good scanner?

 

My 99 s/d had shutter. I had it buzzed and replaced #5 injector. It runs fine now.

 

what is a buzz test?