Keeping cows at a large scale was common, not only in Denmark and southern Sweden (which is the warmest and most fertile part of Scandinavia), but even in Iceland were some farms had up to 60 cows.
So, if the people that inhabited Scandinavia during the Stone Age were lactose intolerant, something drastic evidently must have happened between that time and the Viking Age.
Excavations? My second cousin is married to an ethnic Swede whose family has run a dairy farm for 1000 years. (Supposedly some ancestor went a viking {plundering} and bought land, cattle, and a wife with the proceeds.)