Has anyone ever had an issue with their horses hind fetlock area  "stocking up"  AFTER being worked. The horse is turned out after being worked and does not stand in a stall. Had both stifles injected last season. There is no heat or lameness when this happens. 

Do they go down?  If not, you may be talking about windpuffs?  More information is needed.

Nope. I've only had that swelling appear after my barrel horse was a little older-like 16 or 17. If tied for a couple of hours his back ankles would swell. 

After being worked- are you using boots? Are you just riding or working i.e. asking the horse to sit hard in the ground or small circles etc? 
I would encourage you trying a liniment (after you unsaddle) like draw it out or the corona liniment to see if it would pull that inflammation out. 

Could be too much protein and kidneys overworked in trying to clear the excess.  I'd look at what you're feeding and remove anything that is excessively high in protein.  I have a horse that does this during the winter months when i'm not riding as much if I continue feeding him as though he's getting worked frequently.  I typically pull him off alfalfa or any high protein grain and replace with lower protein, more "filler" type forage and grains with more fat content during the winter.  You may also consider a kidney cleanse type product from Silver Lining or Cavalor.