Adam of Bremen is considered quite untrustworthy in contemporary Swedish historical sciences - there is actually very little Viking Age remains found at Uppsala at all. The great mounds and the great hall are both Migration Period (c.400-600 AD). The Temple has been sought for and never found - actually from what we know the Vikings had no dedicated temples. Their rituals were performed in the open or in the chieftain´s hall.
There are some, a handful perhaps, Viking Age burials which seem to include human sacrifice e.g in Birka but no evidence for stringing up people from trees. On Frösön, Jämtland (Styrbiorn can tell you all about it) a tree with accompanying animal bones has been excavated and interpreted as a Holy Tree.
In the last 20 years there has been a lot of research on the similarities of the Nordic seid (pronounced seith, a sort of magic associated with Odin and Freya) and shamanism. The Saami might just be the connecting link.
BTW the silly little mask were used until exactly 793 when the Vendel period people decided they were actually Vikings and dropped them...
Cheers,
Vandelay