After messing around with regards to this for a bit, I came up with a way in which I can determine the FullExtent of my shapefile.
if (layer is ArcGISDynamicMapServiceLayer)
{
var dynamicLayer = layer as ArcGISDynamicMapServiceLayer;
var featureLayer = new FeatureLayer(new ServiceFeatureTable() {
ServiceUri = dynamicLayer.ServiceUri + "/dynamicLayer",
Source = dynamicLayer.DynamicLayerInfos[0].Source });
await featureLayer.InitializeAsync();
extent = featureLayer.FullExtent;
}
else
{
extent = layer.FullExtent;
}