I don't have experience with DB2 so I can't offer a comparison but I'm, not
sure where your guys would have gotten the idea that PostgreSQL can't do a
high availability setup.
Postgres is sorta like the 'open source Oracle', at least in terms of
features and how they are implemented.
Versions pre 7.1 had all sorts of speed and locking problems but that was
like 6 years ago... We're now up to 8.1 and the thing is simply rock solid.
I suggest that you don't could PG out - it very well may be your solution.