never heard of an oil slinger only thing that should be on the pinion side is the bearings, bearing race's, crush sleeve, seal and the yoke w/nut. unless your talkin about that peice that goes between the yoke and housing on a 9" ford rear
When I replaced the pinion bearing closest to the pinion nut mine had a very flat washer behind the pinion seal A.K.A. (oil slinger). I wouldn't think it would be a problem to run without it but I guess it is possible that gear oil could leak past the pinion seal . Part #13 of the diagram in the link below is the oil slinger.
yes there is an oil slinger see picture below part #16...without this oil slinger you can damage the seal when torqued down which will cause it to leak and i believe it helps in oiling the bearing....are you sure it eas not stuck to the back of the seal...