If it were me, I'd bring it up with my safety committee. Our committee has a meeting every other week, and any driver can attend. In these meetings, something like this would get brought up and our managers would be pressed to explain if this were safe. If not, what else could be done about it? I know many places have worthless safety committees, but that is not the case here. We can actually get things done, because we put them on the spot about any safety issues. And, to our feeder manager's credit, they actually get things done.
Or, maybe talk to your stewards. Good luck with that.
Actually, if those dollies were properly secured, it would seem to be the best option. Chances are, those are red tagged dollies, and no driver should ever pull a red tag dolly. But UPS needs to secure those dollies. If you think that trailer is unsafe, grab a steward and tell them you don't think it is safe and you don't want to pull the trailer. It's not like this is a hot load, and anyway, if it is unsafe, it wouldn't matter if it were hot.