As long as I've owned my car, (4 years) 1st gear always grinds. It does not grind when I try to place it in gear from a complete stop. It will only grind and not go into gear while *rolling*. No other gears have problems. What could be the problem?

I've been told syncro's, but wouldn't the gear pop out while in motion? I need help ASAP, because I have the Tranny out doing a SBC Stage 1 clutch install and it would be much easier to replace while its out if it does have to do with something inside the tranny.