More from the annals of ridiculousness, I bring you a sketch for a “dual differential” swerve.
Unlike other designs, this one lets you set the ratio for steering and drive independently. (the sketch shown has a drive reduction of 8:1, could be 4:1 with a 1:1 bevel, and a steering reduction of ~20:1).
The trick is to modify a couple 4:1 57 sport planetary gearboxes to turn them into differentials. Each differential has two inputs and one output. The top row of gears, red, are connected to the planetary sun gears and spin at speed “A” (where A is the speed of the left motor). The second row of gears, blue, are connected to the plentary housings, spin at speed “B/3” (where B is the speed of the right motor). The output of the left differential spins at A/4 + B/4, and drives the wheels. The output of the right differential spins at A/4 - B/4 and turns the steering gear.
I haven’t put much thought into how the swerve fork is supported. That’s because I’ve taken it as far as I want to go. This was fun to design (for certain definitions of fun), but I don’t think it offers any compelling advantages over other swerve and differential swerves. I am uploading it here only so I have something to show for an afternoon of wasted thinking 
Sources for the general idea:
https://groups.csail.mit.edu/drl/courses/cs54-2001s/dualdiff.html
http://www.technicbricks.com/2008/09/tbs-techtips-17-adder-substractor.html
Differential steering - Wikipedia
