Shudder when accelerating

85 150 short bed single cab. 4" lift. 3.73 aftermarket LSD gears. 33" tires. t18 4spd 300I6

Recently developed a shudder when accelerating. Seems to only happen between 25-50mph and it is independent of engine RPM and transmission gear. The aggressiveness of acceleration does not matter.  It seems dependent only on going through that speed.
When I push in the clutch, it goes away. Even when in the shudder zone.

I am thinking driveline resonance but this is new (it did not exist when my lift was new or when I changed out gearing).

My U joints seem tight.  My front driveshaft has a wiggle in it at the slip joint area and I will probably replace it but that is only a problem when hubs are locked in.  I used to hear a metallic pinging "pop" upon starting out from a stop as a normal sound a while back but it recently went away.

Advice?

This weekend, I'll most likely remove both shafts and replace the U joints anyway.

Is there a specific rear differential angle-driveshaft that I should check?

Thank you!

The angle of the transfer case output and the rear axle pinion need to match. The primary reason is the u-joints do not produce an even rotational speed when they are not perfectly straight so the speed-up/slow-down has to cancel out at the end of the drive train (this is why your front drive shaft has a double joint, called a constant velocity joint at the transfer case as that part is always at an angle).

If the problem did not start right after the lift was done, I would start with the axle to spring attachment and look for any signs of movement (shiny areas). If anything is found there, then it needs to be fixed first.

As for measuring the angles, I picked this up, I think it may have been a Facebook ad, or may have show up on Amazon:  
It works quite well and has a magnetic base.

I have a Wixey digital angle gauge like that in aluminum.
It was about $25 from Rockler woodworking.

I bought it for saw and fence setups but suggested a week ago that Deren use it or something similar to track his home shop caster adjustments.

It helps to check for drivetrain slop when the transmission is in neutral and front wheels blocked.
All other U-joints checked out and were nearly brand new (as I thought they were).  Guess the shop missed this one at the rear diff or deemed its condition as satisfactory. (despite me asking to replace them all while the shafts were out for a different reason)

Looking way back, I can now see the signs of failure from a driving and noise perspective.  It won't get this bad again!  You might be able to see that one of the posts is nearly 1/3 gone.



The truck drives smooth as butter.  I sure would love to have the time and money to drive it to OK for the Bullnose gathering.

Thanks for the input, Gents.

Wow!  That's one of the worst u-joints I've ever seen.  No wonder you had shudder.  

Glad you found it.  But bring it on to the gathering!  

Glad you found it and it was apparently rather obvious once you started looking.