rear end pinion angle issues

[Lookin4the1]

I took my 49 gmc on its maiden voyage today to the muffler shop. on the way back i noticed under acceleration the rear end is literally rotating up to 30 degrees from where it is at stopped. needless to say this scared me to death. I will explain what i've done so far below.

1995 s10 frame
3" Belltech lowering leaf springs
1.5" home made set back plates to center the rear tires in the wheel well.
blisten(sp) drop shocks.

i might need pinion angle shim? whatever it is is scary as hell driving it and it starts vibrating pretty violently around 30mph.