I don't think there's a way to grab the "Last 6 Years" text from the relative filter, but how about finding and showing the Date Range actually represented? The "Last 6 Years" can be misleading anyhow as that is what it allows, not all the dates it actually finds.
So I calculated Min Ship Date and Max Ship Date as Table calculations, and put them in your caption:
Min Ship Date: WINDOW_MIN(MIN([Ship Date]))
Max Ship Date: WINDOW_MAX(MAX([Ship Date]))
Hope this helps!