Error in dsPIC33 Family Reference Manual Oscillator?
You appear to have your \$F_{SYS}\$ and \$F_{OSC}\$ formulae mixed up, per the datasheet you cited.


\$F_{SYS} = F_{IN} \cdot \dfrac{(PLLDIV + 2)}{(PLLPRE + 2)}\$
\$F_{OSC} = F_{IN} \cdot \dfrac{(PLLDIV + 2)}{((PLLPRE + 2) \cdot 2 (PLLPOST + 1))}\$


By my math, with a 20MHz crystal and the following settings, you should get 240MHz at \$F_{SYS}\$ and 120MHz at \$F_{OSC}\$:


PLLPRE: 4
PLLDIV: 70
PLLPOST: 0


\$F_{SYS} = 20MHz \cdot \dfrac{72}{6} = 240MHz\$

\$F_{OSC} = 20MHz \cdot \dfrac{72}{(6 \cdot 2)} = 20MHz \cdot \dfrac{72}{12} = 120MHz\$