between 25 and 60 i have a slight vibration and a humming noise. it stated right after my bj spacer and marlin crawler 6in shackle install. What could be causing the problem ,the u joint, do i need shim the axle to fix the rear pinion angle?

 

sounds like the carrier bearing in your front diff is starting to go... mine started that way. Now my feet go numb around 75... Just gotta find the time to replace my front diff...

That said, I could be completely wrong... YMMV