I am not getting hot water to my showers or any back faucets unless you turn it 'all the way up'. It seems that the cold water starts coming through when switching to shower rather than bath.

It has more to do with the distance from the source of your hot water than a malfunction of your system. You do have to run the colder water in the line out before the hot water can get through. May take longer because of the length to the back fixtures, and more cold water in the longer pipes to those fixtures. The more you open a valve, the quicker the cold water is pushed out until the whole line has hot water. This is pretty normal operations.

Once you get hot water going though, you can turn the volume back down right?

We didn't have this issue a week ago. As long as we have full force ( open all the way) on the hot only, it will stay hot. Once we turn on the shower head ( could be a low-flow head) the water gets cooler and cooler. Turn divert back to the tub and hot resumes. It seems as though the cold water coming in to the heater is going straight out to the system and not letting hot water to get out of the heater. New (2years) heater, Whirlpool. I flushed it and replaced anode rod yesterday, Still some debris in the bottom but only a cup or two of big chunks (left over anode parts).

Then lets start at the basics like name/make and model number for your shower hardware, so we can determine what kind of mixing valve, or shower thermostatic valve you have. Also did you completely flush your shower lines, and clean the shower head with CLR or some other product? You may have two issues to deal with. A valve, and clogged shower head.

Provide the info, and we can find a video of what to do to solve your problem, or you can Google it yourself. That's how I would proceed to nail this down and get a resolution.