I had a similar problem with my 5" springs making my DL angle horrible. So i had a high angle DL made by clearancing an IFS CV for high angle and had that welded to a long slip style DL. I was able to drive about 15-20mph befor it got mad vibration. Then I went and got a high pinion third and I can go 30-40 mph befor i get any wierd vibration. I didnt see if you had dual t-cases or not but that would also help decrease how steep yer DL angle is.
