Maybe someone will know, but I suspect Mcrit for a 737 will be determined by the fuselage. Try a search for Mcrit - there is a bit on PPRune somewhere. I would have a stab at around 0.75M for the 737 Classic wing but that is a wild stab - Mcrit is very difficult to determine. Mcdr is easier from fuel flow tables.
Here is one thread which quotes Mcrit for a DC9 as 0.73M.