Here we are in 2017, about 7 years on, and I gave finally gotten around to joining the Litvak SIG of JewishGen.org.
Sadly, there in the All Lithuanian Database I found voter registration records for Sholom Berko Krok, his wife, Etel and daughters Stira Yudes and Libe Raykha -- as well as his 77 year old mother, Chava. They are dated 1908! This seems to put an end to Justin's Krok hypothesis.
I enjoyed having it for 7 years or so. And NOW I find that at least one of my 3 --4th DNA "cousins" has Kroks in his family tree. However, Krok (present day Krakes) is a village in the right area and it is possible that its inhabitants took Krok as a surname for use outside the village.
Cheers,
Geoff