I didnt touch the drive shaft. Wheels and U bolts are nice and tight. The forward spring eye bolt is torqued to 150 because that is as high as my wrench goes. I saw one spec that said 220 but that seems ridiculous.

 

Sorry brother, I must have missed that. If it's only at a certain speed then I think it must be something else. Hard to say what. Have you tried it with a load in the back?

 

Maybe you put the new springs in backwards?

It would cause an odd pinion angle and could cause shudder.

 

I haven't tried it with a load in the back yet. Ill give it a shot this weekend and see what happens.

 

The springs are in the right way. They only fit one way because they come with the bushings already intact and the bolt sizes differ from the front and rear.

 

Yes, the rear shackles(hangers) need to move,when the spring flexes upward.
In your picture is there a space between the spring and the shackle? - the bolt that is touching the frame. It looks like the shaft of the bolt showing, does the spring slide left to right?

 

How did the bolts slide through the springs eyelets, easy or hard?
I'm thinking something is too tight or too loose, or maybe one of your springs has some slight warping to it.... The best way to check the springs would be to remove them, slide a bolt through their eyelets and connect them together parallel... Once they are side by side you can observe any length issues, twisting, and maybe even contrasting spring rates (one may be stiffer then the other).
Either way you should pull em apart and grease the crap out of the bushings, bolts, leaf shims, and sand down and clean, clean, clean, the block to perch and block to spring mounting faces...
You could also upgrade your bushings to urethane.
Keep us posted, I want to order springs from these guys too, as long as there quality.

 

They don't slide side to side the space is just from the way the bushings stick out a bit. I turned that bolt around so it's facing the same way. Everything is tightened to spec.

 

At this point i would rather live with the shudder than pull it all apart again. However, i will take your suggestion and pull them apart and check everything because i dont know what else to try. The bolts went through fine but it couldnt hurt to clean the blocks and the holes like you said.

 

Also im worried about the axle rotating on me when i have both springs off. Whats the best way to avoid this?