Hey Andy,
The bearings should not get difficult to turn when you tighten the pinion gear bolt. If this happens then the bearing inner races are getting preloaded, probably due to the inner race protruding up too far on one or both of the bearings.
If this happens, take the bearings out, flip one of the bearings 180 degrees, and reassemble the into the gearcase. This will change the bias of the inner races, which should solve the problem.
Then you need to make sure that the gearcase doesn't become difficult to turn when you bolt the rear hub onto the front face with the six cap screws. If this happens, then the problem is still there, and we might need to send you replacement bearings.
Whatever you do, do not fly the Drive if the unit tightens when you assemble the front hub, or tighten the pinion gear bolt. The Drive should always rotate freely.
Brenner ...