What is the difference between a 1999 freightliner classic vs 2000 freightliner classic?

If you're talking about two trucks which were manufactured in the same configuration with matching specifications, date of manufacture would be the only difference. They'll both have pre-ACERT motors in them, although I believe 2000 was the year that Caterpillar increased the displacement of the 3406E motor to 14.6 litres, so that may be a difference if they have Cat motors.