The Gymkahana Suspension is an after market anti-sway bar package developed by Larry Mitchell and some AMC autocrossers.
They've been around awhile and work very well. Many AMCs use the Gymkahana Suspension successfully in racing including Tony Zamisch's 1970 Javelin 'Great White', which currently holds the AMC Land Speed Record read more.
I have one on my 69 AMX and would recommend them to anyone wanting to upgrade their suspension.
You can read more about the Gymkahana Suspension by clicking here: AMX Enterprises
Larry still sells these bar packs.
On a side note: there are other sway bar kits available but I believe the Gymkahana Suspension is the only one developed and sold exclusively by an AMC enthusiast.