Summoning the almighty /u/EyeStache & /u/textandtrowel from Valhalla, hopefully they'll be able to provide insight into your question as I'm intrigued as well :)

Sorry about the delay in getting to you, but I was out of town and didn't have access to any sources.

Okay, the short answer is - as with so much early medieval Scandinavian history - we're not sure, but probably maybe yes.

See, we have a description of them in Gesta Danorum, which says that, after the town of Julinium was taken in the 10th century, a bunch of badasses were given as a gift to Styrbjǫrn sterki, an exiled prince of Sweden, by Haraldr blátǫnn, king of Denmark. Syrbjǫrn then went about generally terrorizing the seas with his viking band, and in 985 fought a disastrous battle against his cousin, Eiríkr inn sigrsæli, where he was killed and his band was scattered.

Now, Gesta was written about 200 years after the fact, and is our earliest source for anything that could potentially be the Jómsvíkingar, as the next source we have mentioning them is Knýtlinga saga, written around 1250, and agrees with Saxo's account in Gesta. After that, we get into myth (Jómsvíkinga saga has a guy named Palnatoke founding them after a mythical Wendish king named Burislav gives them Jómsbǫrg) or later texts (like Eyrbyggja saga) agreeing with Knýtlinga and Gesta.

So, while there probably did exist a fortress from which víkingar raided and were incredibly effective (until 985, at least) there's no evidence for them existing before the mid-10th century, nor for their existing beyond that.

So, again, probably maybe yes.