Speaking for UCSD, junior transfers have litte extra difficulty with rush. Before anyone asks, the rush apps that I saw distinguished between a 3rd year and 3rd year transfer student.
Juniors are generally fairly successful with recruitment at UC schools. Many of the UCs have junior quotas and those who do not still place a large percentage of the juniors who participate in formal recruitment. It is normal to find a large number of sophomores and juniors participating in recruitment on these campuses in general. This is also true at USC, but to a lesser degree.
Back in the day at USC we had a specific number of Juniors that we were able to take above our other quota. Basically, a junior quota. It wasn't very high but I did know a lot of juniors who didn't have a problem getting into the sorority of their choice . . . both transfers and juniors who had waited to rush.
Here at UC Davis we don't judge on grade. In fact, we will take Seniors, too. We judge on the personality only. Actually, we have a rule in my sorority that we can't judge on grade level, but we should have pay special attention to freshmen since they will be around the longest. It is a really popular option to transfer from a community college or junior college in CA and rush as a junior.
At Berkeley, Juniors don't count towards quota (only freshmen and sophomores do) so it is definitely not more difficult to receive a bid. I know at least a few girls who pledged as juniors. It definitely is more about personality and whether the girl clicks with a sorority than anything.
Juniors (including junior transfers) at UC Riverside tend to have the same chances as any other student. They do count toward quota still, but as there are really not that many of them that go through recruitment, it isn't often a problem for them.
I'd say the most competitive rush in Calif. is at USC, which is not a UC school. I wouldn't say the UC schools have extremely competitive recruitments where grade would hurt you...in fact, as others have said, being a junior on a campus with "free juniors" might actually be a good thing.