The concept is to have the medium-caliber, medium range EFSS covering the direct support work for the fast-moving maneuver units, the LW 155s providing the battlefield shaping/reinforcing/counterfire missions and the HIMARS (High Mobility Artillery Rocket System) providing the long range high value target/counterfire punch.
The EFSS is aimed at supporting the deployed MEUs with something a bit more portable (the 155s aboard ship are referred to as "ballast"). Depending on the mission and the forces we're dealing with, we will bring in the heavy hitters for well-balanced fire support.
The Russians have come up with a very neat little ATV-type forklift for use with their 152mm gun-howitzer ammo. And it can double as a prime mover for the wheeled guns for short local movement, as well.
The US built Groundhog ATV forklift [or bucket blade] is similar, and could likely be arranged to work with a skid-steer tracked Bobcat Model 864 vehicle instead, or besides. About $2500 for the Groundhog ATV, and $379.00 for the forklift option.