XAS spectroscopy - Differences in magnitude of 2p SOC and 3d SOC?
The energy of a spin orbit level for a '1 electron' or 'hydrogenic' atom such as sodium is given by

$$E_{so}=\frac{AZ^4}{n^3(l(l+1)(l+1/2))}\frac{j(j+1-l(l+1)-s(s+1)}{2}$$

where $A$ is a collection of constants and $j,\,l,\,s$ quantum numbers derived from the term symbols for the levels involved. 

The second fraction in the equation is the angular momentum term, the first due to $\langle1/r^3 \rangle$ where $r$ is the distance from the nucleus. 

For a D electron you might have $j$ quantum numbers $j$=5/2 and 3/2 with $l$=2 and $s$=1/2 and for a P,  $j$=3/2 and 1/2 with $l$=1 and $s$=1/2 thus the separation of the D level pair is less than that of the P.

Most good textbooks will have a derivation of this equation and other corrections such as due to relativity etc.