I have a 99 xcab with 373's. Will a rear sway bar off of a 99 explorer bolt right up to my rear end??

No, Explorers use a completely different rear sway bar setup.

Bronco II works with my 95'. Might work for 99', I think the rear is the same.

Any thing from 1998 to 2004 rear bar will fit. Only 2003/4 bar is R/C 4x4. You will lessen rear roll but will have to watch higher spd turns it will tend to oversteer possibly, that is what bars do.

Are you trying to fix something?