need a extension replaced on a carbine barrel, and headspaced.  m4 extension would be nice.  Anyone know who can do this.

Don't bother. It's really alot of work to change the barrel extension an it is going to cost alot more in labor and machine work to do it right than just buying a new m4 barrel assembly ready to install on your upper.

It can be done, but it could lead to alot of work.  The gas port is indexed to the barrel extension, so you would have to be lucky enough to find an extension with the exact same thread start to have it be tight and exactly indexed.  I've tried it once and got lucky, but I had in excess of 30 extensions to choose from and all but one of them wouldn't have worked.  You can tinker around with the shoulders of the barrel and extension to index it properly, but this can throw headspace way off, and you can't adjust headspace by reaming the chamber on a chrome lined barrel, but you can with a chrome moly barrel.  A new barrel is generally in order for this.

Quoted: I've tried it once and got lucky, but I had in excess of 30 extensions to choose from and all but one of them wouldn't have worked.

Where is the icon that shows me bowing and scraping at your feet...

I can't say that I have ever changed one, and I can't say that I ever will.  Like you said, a new barrel seems to be a better beast.