Vibration after spring change

I have a 71' dart 225/904/7.25 I just changed the leaf springs and now I'm getting a vibration while driving. I rechecked everything with the springs and it all looks good I'm thinking I may have damaged something else while doing the change. The vibration starts at 10mph but goes away if I shift to neutral or if I get up to 15-20mph. Then it comes back around 30-40mph but it's more of a drone then a shaking. At 10mph it shakes the car but at 30-40 it's a lot less noticeable. I'm thinking u-joints but I'm not sure how to check those. Any ideas? I've been playing with this for a few days and can't figure it out.