My PRO2 plunger is not traveling smoothly through the brew head, or I'm feeling a lot of resistance.

With use, the plunger will naturally lubricate thanks to the exposure to coffee oils. However, you can expedite this process by applying a light application of food-safe oil (i.e vegetable oil) to the plunger o-ring and wiping away the excess. It is also good practice to do so after cleaning the plunger, as you will be removing those oils once again. 

However, if the plunger/stem is not moving down through the brew head without the application of excessive force, and it feels more like a blockage than a lubrication concern, it is more likely an issue with your grind size than the machine/its components. If you are grinding too fine, water will be unable to pass through the coffee bed under normal force, thus building up this pressure/resistance in the brew head and preventing the plunger/stem from lowering. 

Carefully terminate your attempt by slowly raising the lever (do not abruptly let go: the pressure built-up will result in a rebound effect, causing the portafilter and cylinder to separate) and reattempt with a coarser grind size. Never apply excess force/attempt to force the lever down if too much resistance is met. A proper shot brewed at 6-8 BAR translates to about 40-50 lbs of force on the lever.