And also remember the V8 has the high torque further up in the rev range. The TDi gives high torque rather low, so you'd be hitting max torque nearly all the time with normal driving unless you're very very very good, and therefore likely to rip the series transmission to bits.
If you did what Redlinemike said, then you would have to change the swivels and other parts in the process. Series Land Rover swivels are Universal Joints, while all coil sprung Land Rover swivels are Constant Velocity joints, two very differant things. Series Swivel UJs cannot take permament 4WD without running the risk of eventual failure.