my chevy g20 dayvan shakes at 35mph

hi all im a newbie
i drive a chevrolet g20 dayvan (only owned fri 5 of sept) 5.7 v8 1988 model and around 35/40 mph the van shakes. When you put accelerator down it stops shaking for about 20 seconds then starts again and when i brake it still shakes till around 20 mph. When i brought the van i was told the carb needs looking at would this mean that the two are related ??? also every now and then the service engine light comes on sometimes straight away when van starts other times within 10 mins of a journey. Thanks for taking your time to read