Traxxas center diff
 

I was thinking about putting a center diff on my e-slayer but had a few questions. Does it go in the actual diff housing? If so then don't i need two for a 4wd truck?

I'm not super familiar with the center diff conversion, but it goes in the transmission. You already have differentials in the front and rear :) Hence, this is the center diff; goes in between the front and rear diffs. It allows you to tune the power delivery and adjust how much power goes the front wheels relative to the rear wheels.