hey guys..nebie here..picked up a 97 g3500 van to haul my motocross bikes in....runs great..but about 55mph when the van shifts into overdrive i think it sort of rumbles a bit..like on rumble strips but not nears as bad..just a mild shake when it shifts..just when it shifts around 55...other than that its strong..any ideas>?..bout rying an additive?..thanks