Yep....overtightening the pivot ball covers will definitely seize the suspension movement right up
One other option for freeing up the NTC3 is to "modify" the diff cases.
The bearings for these "snap" into place and when the cases are assembled and tightened, there will be very slight binding on the bearings.
To free this up, each half of the case needs to modified slightly using a hobby knife.....the recessed plastic area of the case where the bearing sits can have some material removed to free up the bearing. Remove only very very slight amounts of material at at time. Put the bearing in the case, the turn that half of the diff case upside down.....when the bearing falls out of its own weight the job is done.....the secret is dont remove too much material from the cases
Cheers