DARLINGTON, S.C. (AP) — Dale Earnhardt Jr. is grateful to be back at Darlington Raceway preparing to drive two weeks after he and his family escaped a plane in flames following a crash in Tennessee. Earnhardt, set to drive the Xfinity Series event here Saturday, said he and his wife Amy are still processing the frightening events Aug. 15 at the airstrip in Elizabethton, where he had flown to…