As far as I know, that prototype '69 Eliminator has not surfaced so far.
Besides the Dana-Spicer two speed rear axle, the molded in rear spoiler, the Torque-Thrusts and the paint also didn't make it to production.
Mercury branded that rear end as "Streep", for street and strip and they launched a whole "Streep Scene" marketing campaign around that, creating a logo and some of the wildest and best advertising materials of the late '60s.
So I would think that that two speed rear axle was seriously planned for production up until the very end - to late to alter the marketing campaign.
Phillip