To install a Quaife carrier into a 215mm rear, you are looking at around $1400 for just the unit.
Then the rear has to be removed to install.
That means axle shafts have to be pulled also.
Then tear into the rear to install the Quaife.
Hope pre-load and backlash are OK as there is no adjustments.
And this cost is based on using the same gears.
Add 3:55 Richmond gears for $500 more.
Now you have to set-up the pinion gear also.
For around $1500-$1600 you can purchase the Getrag 226mm rear, axles, & hubs.
You would have a beefier rear with stronger axles and Limited Slip.
Plus you could get 3:73 or 3:91 gears already installed in the unit.
A complete bolt in set-up.
You could then sell the 215mm rear, axles, and hubs to a LX owner for about $500.
This is an upgrade to their units.
As far as the gear ratio change, a Predator is capable of making the different change to the PCM.
I have the Getrag 3:91 in my auto and they are quite low of a ratio.
But I race mine.
For the street, the 3:73 are a better choice.
Brad