What main landing gear actuator does the Boeing 777-300ER have?
The Boeing 777-300ER is the only variant of the 777, which has a Semi-Levered Gear. From the 777 FCOM (v2 14.20.3 Landing Gear - System Description):

Semi-Levered Gear
[777-300ER]
The semi-levered gear consists of an additional hydraulic actuator that connects the forward end of each main gear truck to the shock strut. During takeoff, the actuator locks to restrict rotation of the main gear truck and allow takeoff rotation about the aft wheel axle, thereby improving airplane performance capability. During landing, the actuator is unlocked to permit rotation of the main gear truck and provide additional damping.

The 777-300 and 777-300ER are both equally long at $ 73.86 \, \mathrm{m} $, which is about $ 10 \, \mathrm{m} $ longer than the -200 variants. Due to the higher risk of tailstrikes, both variants are equipped with a Tail Skid:

Tail Skid
[777-300 and 777-300ER]
The airplane is equipped with a tail skid system. The tail skid extends for takeoff and landing and retracts during flight. It helps protect the pressurized part of the airplane from contact with the runway. The tail skid uses the main lading gear actuation system.
The EICAS advisory message TAIL SKID is displayed when the tail skid is not in the correct position.

The -300ER however has a MTOW (maximum takeoff weight) of $ 352 \, \mathrm{t} $ compared to only $ 299 \, \mathrm{t} $ for the -300, which increases the risk of a tail strike during takeoff rotation.
This Boeing article explains it as follows:

The Boeing 777-300ER's new semi-levered landing gear system has performed flawlessly during the flight-test program. The unique gear, which is manufactured by Goodrich Corp., allows the airplane to rotate early by shifting the center of rotation from the main axle to aft axle of the three-axle landing gear truck. As the airplane rotates, the nose is allowed to rise higher earlier.

You can see the difference here:

(image source: airliners.net and youtube.com)