Truck starts to shake when going 25-30mph???

[wldcat10]

Ok I have a 2 problems. (#1) I was driving my '70 lwb c/10 on saturday and it started to shake when I was going about 25-30mph. I pulled over and couldn't really figure it out what was wrong? So I got back in and did the same thing again around 25-30mph, if I speed up it kind of lets off and if I slow down the shaking slowly starts to fade out. Not sure what it could be? Could it be that my shocks need replacing? (#2) Noticed that a had a small spot on the concrete where something was leaking under my truck. It was a very small leak from the speedometer cable was. So I tightened it and it stopped. Now my speedometer needle jumps back and forth between 30-50mph on the gauge. What can I do/adjust so that it doesnt leak and I dont have the speedometer needle jumping all over the place. Any help is appreciated, thanks!!!