Barb from Regina has a house with a slab on grade with the furnace in a closet and the heating ducts running under the concrete and the cold air return high on an inside wall. Some of the ducts don't give her much heat and the floor is often cold.
High inside wall heating and return ducts were once common in the Prairies and they worked when we had very powerful furnaces moving great quantities of air. Today, with all of our energy conservation, we are moving less air, and these old arrangements don't work well.
First she should re-route the cold air return so that is will draw the cold air off of the floor. Then close off the duct dampers, or in her case where there are no duct dampers, close off the floor grills in the rooms that have plenty of heat. This will force more heat into the remaining ducts. If that doesn't do it you will need a booster fan to help out the back bedrooms. I recently saw one that will be on the market shortly that installs right over the floor grill itself.