99 Blazer engine interchange

I need an engine for a 99 Blazer 4X4, 4.3 Vortec. Im having trouble finding one because everyone tells me that only years 99 and 2000 will work. 98 and under engines are plentiful Does anyone know the difference between a 99 and a 98? What would I have to do to make a 98 work?

Anybody??