NEED HELP Random Downshift Thud

[therealsethallen]

I've got this random driveline thud. Labor Day weekend I replaced my rear U-Joint and diff fluid with Synthetic 75W-90 and a Friction Modifier. Anyways, in the last few weeks I've had this weird downshift thud, it's only happened twice, and once when turning the engine over.

Both times it happened when moving, I was slowing for a light, and to make my street. The thud sounded like it was directly under my knees. A little in front of the pedals. I'm wondering if it might be the front driveshaft, or it's U-Joint, which I didn't replace. It's a pretty harsh thud. Another possibly related issue, the truck jerks on downshift deceleration, it almost feels like the downhill jerk in cruise before the last software update. It pushes the truck and I have to apply a bit extra brake pressure. I'm stumped. Everything looks fine, the truck runs the gears just fine, it starts fine in the morning and does my daily commute issue free.

 

[Sparky]

What about the forward U joint on the slip yoke? Or, I recall some people having issue with the slip yoke sticking and clunking when it would free up. Some reported it being like a bump from behind feel. Pulling it out and greasing the splines seemed to help that (and then top off the transfer case fluid since some will run out when you pull the slip yoke out of the tail shaft of the transfer case).

 

[TollKeeper]

What sparky said. I think it's drive line. my truck used to do it to.

 

[therealsethallen]

The T-Case is about due for a fluid change, while I do that I'll change out the front joint and grease the slip yoke. I've got some lift time free tomorrow, I might jack it up and see what's going on. I might just get a new driveshaft for the hell of it. But my front end it the main concern, the control arms are wasted, and I'm sure that it's got to do with it.

 

[therealsethallen]

Update. I checked out the front driveshaft and it certainly needs a U-Joint, though I suspect that isn't the whole of the issue. I did notice today that the seal around the driver's side CV shaft is leaking fluid quite badly. I knew it was leaking, but it's gotten a lot worse lately.

Secondly, it did thud again today, it was more pronounced, too. Actually sounded like it was coming from the transfer case. I'm not ready to say it was in the gearbox yet and it really doesn't feel that way because it shifts just fine and the fluid looked fine at last checkup last week. Not sure what the actual job is for lubing the slip yoke.

 

[Sparky]

Pull the shaft, grease the splines, shove it back in. Pretty simple.

 

[therealsethallen]

Well for me it'll be pull the shaft, install new U-Joint, grease the splines and shove it back in. But I do get the picture.