Zero field muon spin relaxation (ZF-SR) has been used to study the magnetic properties of the underdoped giant magnetoresistive ruthenocuprates RuSr 2Nd1.8-xY0.2CexCu2O 10-δ (x=0.95, 0.80). The magnetoresistance (MR) is defined so that MR=((H0)/0) and the giant magnetoresistive ruthenocuprates RuSr2Nd1.8-xY0.2Ce xCu2O10-δ exhibit a large reduction in electronic resistivity upon application of a magnetic field. The ZF-SR results show a gradual loss of initial asymmetry A0 at the ruthenium spin transition temperature, TRu. At the same time the electronic relaxation rate, , shows a gradual increase with decreasing temperature below TRu. These results have been interpreted as evidence for Cu spin cluster formation below TRu. These magnetically ordered clusters grow as the temperature is decreased thus causing the initial asymmetry to decrease slowly. Giant magnetoresistance is observed over a wide temperature range in the materials studied and the magnitude increases as the temperature is reduced from TRu to 4K which suggests a relation between Cu spin cluster size and |MR|.