The "True Force" as i call it, has been underlying Star Wars for the longest time, but has only been recently revealed (and only a little bit) through Jacen Solo in the latest New Jedi Order book, "Traitor". Spoilers will ensue...
Basically, the "True Force" is part of everything together. However, there is NO "Light side" or "Dark Side" to it. The primary difference is that "Light Side" powers are derived from the Force, while "Dark Side" Powers are derived from within the forcewielder.
Long Long before Yoda or even Exar Kun, probably around the beginning of the Jedi, The Jedi themselves created the distinction of "Light" and "Dark" sides to the force. They did this So they could have better control over it's use. Maybe their intentions were good, (Since the Force Within is always muddled by emotions) but every single Jedi that has come afterwards has grown up using the force with the wrong mindset, never using it to it's full extent.
So, to answer your query, Neither Exar Kun nor Yoda used the "True Force" but merely used an aspect of it... Mostly because of what they were taught. I'm pretty sure that there has never been a single Jedi that has tapped into the full extent of the True Force; The Unifying Force.
The NJO Jedi actually have pretty much figured it out. Kyp Durron believes in the power of the Force Within, while Luke Believes in the Binding Force. Problem is, neither of those guys has figured that BOTH of their ideas, working side by side, is the Force as it has always been.