Yes, as far as I know if you purchased them yourself way back when they had lifetime warranties, the warranty should still be still in force.
However, since the knobs are available separately I believe, it may not constitute a warranteeable offense to replace the entire shock. They can just send out new knob assemblies and you can attach them yourself.
Even the bushings used to be covered, but I believe that they nixed that years ago for those that still had lifetime warranties. The bushings and boots are "wear items" instead.
Of course, shocks are wear items too! Anything "lifetime" was a gamble for the manufacturer, hoping you would not keep the vehicle that long. A friend of mine still has his lifetime JC Penny battery warrantied every few years in his '73 C5 Blazer.
I forget who does the coverage (Goodyear maybe?) but they give him a new battery every few years. No longer the 1,000CCA unit that JC Penny marketed, but hey, even a wimpy three-year battery for free ain't half bad.
I'm not sure how the new/current shock warranties are played, but they have certainly made changes over the years.
Might call Rancho customer service to see if anyone there is old enough to remember how it works!
Paul