Yeah, the Cusco is an open diff.. it has a fixed split and doesn't rely on friction internally (such as a plated or viscous LSD) to vary the torque split front to rear, its all set in the planetary gears.

Everything I've read says you can dyno cars on Dynapacks with varying torque splits front to rear.. it'll even give you power/torque readings for the individual axles!! Also, you can switch 4wd to 2wd and vice versa on the fly and the Dynapack will handle it.

If thats the case, it must be something odd with the Tarmac Gear that the dyno may not be able to compute.