I have a 86 3/4 ton GMC.  Raised the front 4 inches with a block. Now it chases ruts, darts and causes white knuckles from holding onto the wheel.  I have replaced shocks and streering damper.  What's going on?  Thanks  

You lifted the front of  your truck with a block, that is what is wrong.

Your steering angles are probably all screwed up.  

I'd get them blocks out of the front quick if I were you! I had some idiot mechanic recommend the same thing for one of my beasts. I knew he was full of *** to even say such a thing. If you are using those cheap auto part aluminum blocks - they are prone to disintegrating on you. This arrangement will get you into: A - a rollover when one of them blocks gives way, or B - a head on collision.
Blocks are okay for the back end, although I'd make sure they were heavy cast iron ones, but if you want to lift the front get add-a leafs for a smaller lift or replace the front spring packs. They're not that expensive unless you're looking for 52' inch leaves or a huge lift. Check out Rough Country or High Jacker. Ranchos are cheap but stiff as *** and will rattle your teeth whne you get into the hard stuff.
Get them blocks outa that front end!!
The life you save may be your own - not to mention the other folks you may hit.  

Well said Red!

Block are BAD in the front! Can't stress that enough!