The nitrogen cells are at the end of those lines from the struts, as you thought. On the wagon, they are replaced from inside the car. The load floor immediately behind the backseat is removed; the cells are located beneath it. It's been a few years; so the exact procedure is fuzzy in my mind. However, there are about six or eight screws to remove. The seat back of the third seat may need to be removed - don't recall for sure. Two of the screws are in the load floor a few inches behind the backseat. The remaining screws are in the lip of the floor such that they are hidden by the third seat seatback when it is down.
Replacement is quite simple. Bleed pressure out of system. Remove old cells (three nuts, one pipe fitting each). Fit new cells. The tricky part is the hydraulic fittings all use straight cut threads - it can be quite tedious to get the threads started, given the constrained workspace. No bleeding necessary.
The nitrogen cells run ~$80/each. Check prices at Fastlane here. (OK, I remembered to put in a plug...)
I'm surprised your front struts feel worn out at only 120K miles. Typically shocks/struts on an MB last much longer than that. More than 200K is typical of shocks. My own '87 wagon has about the same milage and has quite firm damping.