Rear-end rebuilt, but now getting intermittent strong shudder when moving out of park

[MikeCA]

Here's an update to my rear-end rebuild.

I've had the rear-end rebuilt and the truck now flies with literally no vibration at 65+ (the speed at which it would begin) other than what I would attribute to the tires (over-size BFG AT KO's) and perhaps slight pitting on one or two wheels. It's smooth as butter by comparison to what was there before.

BUT, since the job was done on a couple of occasions I have felt a sudden and sharp vibration from what I would attribute to be the rear-end. It lasts at most 2 seconds and occurs when:
- moving forward from park, only happens with wheels turned from the start
- slow speed
- flat surface
- shudder dissipates within 1-2 seconds, quite possibly as wheels are straightened out, can't remember
- no other vibration is felt while driving at all speeds, no other vibration while turning at slow speeds from four-way stops, etc.
- occurs randomly so I've had difficulty replicating this to gauge where it might be originating from, but it definitely feels like it's coming from the butt-end

Any idea if this is something to do with the rebuild just getting worn in or has something been botched by the shop? As I've mentioned before, the rebuild is working really well with no problems other than this shudder that's come up 3-4 times since the job was completed just before Xmas.

Thanks in advance.

 

[TechGuru]

Is this a 4x4?

 

[MikeCA]

Awd.

 

[MikeCA]

Major revelation today, I got this shudder twice now while in park after having started the engine. It also happened while moving slowly with wheels straight ahead.

So two times while in park (what the...?) and once when moving forward leads me to believe this has nothing to do with the rear diff, and the tranny is looking like a definite unlikely source as well.

Why would the vehicle violently shudder like that for 1-2 seconds while in park just after starting the engine? That shouldn't be an engine mount gone bad, shouldn't be a vacuum leak. What is this?

 

[boominXplorer]

Is it limited slip? If so make sure you are using 75-140 gear oil with a bottle of friction modifier from Ford.

 

[MikeCA]

But why would the vibration occur when in park, and only a short moment after starting the engine and only that ine time, if at all. Not every start has this issue, maybe 1/6 or so.

No error codes via my ScanTool MX.

 

[boominXplorer]

If it started after the diff work it most likely has something to do with it. Your right though if it has done it while sitting still then it's not the difference fluid. Check the exhaust hangers.

 

[MikeCA]

Yup, just by fluke I decided to start the engine and just idle for a bit while checking my phone and there went the shudder/vibration. It's crazy, but lasts a short time. So you could be right, the frequency of the exhaust right at that time causes a vibration as the engine goes through its startup RPM ramge.