I didn't notice it before my tune because I couldn't go over 90mph...silly GM for governing it. But as soon as I got my tune back and went over 90mph, I got a nasty vibration from the rear end. At about 92mph, it begins; 95mph you can definitely hear it and slightly feel it; at 100mph, its really loud and there's an unmistakable feel of vibration, and I haven't gone any faster than that.
I didn't have my drop shackles in when it first happened, so they didn't cause it; this vibration was part of the truck since I bought it. I thought the shackles might have some effect on the vibration...but I was wrong. The vibration doesn't seem any stronger or weaker now than it did before.
I read a little bit into adjusting pinion angle... What do I need to do to fix this?

 

All right, not a clue.

 

Do you have stock rims and tires?

 

+1^ check your tires balance, have them road force balanced (i think thats what its called)

 

thats strange that your truck before the tune govern'd at 90 most are at 98 or so...maybe a different tire size on there throwing off the speedo?

 

Nope, all stock rims and tires, straight from the factory. I thought it might be the crappy general tires, but I won't know that unless I replace them.
What is a road force tire balance?

 

Ok, update... I brought my truck in for a road force tire balance. They ran through it all and the tech said the balance on my tires was pretty bad, but all is well now. So I get on the highway and bring it up to 95...vibration is still there, strong as ever. So now we know it's not my tires, despite how shitty they are.

 

stop bringing it up to 95, problem solved

 

How dare you say such a thing. lol

 

1st what are the tires rated for?Speed wise
2nd To correct the pinion angle, you will need to add shims on the spring pack perches.