The poll which found 56% of college freshmen in fall 2000 and 58% of college freshmen in fall 2001 supporting legalized gay marriage was conducted by a Higher Education Institute survey of 269,413 freshman students at colleges and universities nationwide.
That number refers to the '01 poll; sorry, I don't have a weblink of the full poll. The margin of error is less than 1% (probably near zero, considering the huge sample).
In mid 2000, ICR of Media PA conducted a poll which was commissioned by the Associated Press that found 54% of those between the ages of 18 and 34 supported legalized gay marriage. ICR polled 1,012 adults between those ages with a margin of error of +/- 3%.