When you say the steering gets 'tight', I take it you mean that you have to fight the wheel to keep it straight?
Your tires are inflated and balanced correctly? (Although your problem would not be intermittent if that was the case, but check it anyway).
It could also be that your discs in the font are warped but usually your steering is not really affected. Does the vibration stop or get worse when you press your brake?
Perhaps the brake caliper is not properly floating on the assembly. Disc brake pads (mounted on the caliper) are meant float freely around the disc. It could be that dirt or rust got into the assembly and this doesn't happen as freely anymore.
Take your car to an empty parking lot and make tight turns, turning the wheel as far as it will go. Listen for 'clicking' sort of sounds -- it could be that one of your U-joints is bad (probably your right one). I had a similar problem years ago (non Nissan) where it almost seemed like an invisible passenger jerked the steering wheel one way.
These are just some possible things you can look at -- others here will have more things to try, I am sure.
Remco