Why can you only get full beam on your 90 300zx headlights?

By Full Beam I take it you mean High Beam.

Because the low beams are burned out replace the bulbs.

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This could be a possibility but it's very unlikely for two bulbs to burn out at the same time. you may also notice your side marker lights dont work as well. if this is the case then it is the fuse. not the ones in the engine bay but the hidden fuse panel just to the left of the clutch pedal. one you have located the fuse panel check the one for the ones for the headlights. Also if you are having this problem and your car has running lights the you want to pull the headlight switch assembly out of the dash. Once you've done that you will want to disconnect the wire that goes into the running lights. It's very easy and once you have done this you will never have a problem with your lights again.

hope this was helpful for someone. it's a very common issue and the fuses are the easiest fix but usually overlooked.