The 2.6 has a pretty long stroke for low end torque. A long stroke gets you higher piston speeds and makes the 4 cyl sahke problem worse.

Once they had a design with the ballance shafts having light pistons and rods as not as much of an issue and a long stroke 4 cyl was as smooth as a six cyl... So the 2.6 gets some pretty beefy parts and a long stroke for good torque.

Want to do a 4 cyl without ballance shafts? Use a short stroke, light rods and pistons and tune the major components that are bolted to the motor to have a high resonant frequency.

Kevin