Have you tried locking only one hub, and seeing if turns as easily as w/both unlocked?
If it turns both direction easily with only one hub locked(test both sides locked separately), then the issue is the welded front diff, either by itself or in combination with the rear.

When turning the inside tire turns slower than the outside one, locked diffs want to push you straight forward, you are fighting two issues with welded diffs when wanting to turn.