The rear wheels are now locked together, and cannot turn at different speeds.

If one tyre is underinflated or has a lot less tread, then the rear will try to turn a corner all the time.

If you swap the rear wheels over, it should pull the other way.

Also check bushes on rear arms, as if there is a lot of play in them the rear can steer the car.

Also check front end, as it may have nothing to do with the diff work.

An LSD would have been a much better solution.