we did this in a charger so i know it will fit.

just in case you want a 3.91
first you get a rear,3.91 posi unit from a challenger off ebay for around $800.00 also
get the hubs and the axles,
from a cls get the rear mount bracket
drill a hole in your rear mount support for the new bracket for the rear to fit
replace the rear hubs, or re spline the axles for the crossfire hubs
call Rudy for the TCU

your done.