same-difference substances

Those were the days when he took off with Silver, who had also returned to Australia, to spend Christmas with friends in Adelaide, before rejoining Bon in Sydney in January. With Silver soon establishing good connections in Sydney and Bon drinking more heavily than ever, they made an odd pair; both stoned on same-difference substances. ‘We hired a little motorcycle so we could get around,’ she recalled. ‘There were a few nights where I’m trying to go down the [. . .] expressway and he’s on the back swaying away.’

1 What does same-difference substances mean?

2. I’m trying to go down the [. . .] expressway and he’s on the back swaying away.- So she is going to the subway and what is he doing in other words?