Lehman Brothers easily moved to the top of the list of private-label MBS underwriters in 2000 with $12.88 billion in volume and a healthy 18.3 percent market share, a year-end ranking by Inside MBS & ABS reveals. Lehman jumped from fourth place in 1999 to take the top spot last year with a 27.6 percent increase in business. In fact, Lehman literally traded places with Donaldson Lufkin Jenrette, which fell from the number