Currently, UPS workers can unionize in local divisions, say individual sorting locations or maybe the employees in one state. However, FedEx employees can only unionize nationally, all 125,000 US employees, or none of them.
It's really murky, FedEx was founded as an airline and operates the largest commercial air fleet in the world, so they've been regulated like an airline. But they also operate an even larger trucking fleet than UPS, which is regulated like a trucking company. And UPS operates a huge air fleet too. I think the key to me is that FedEx operates their trucking and airline as separate business entities, but that may just muddy the waters even more.
All that, and what is mentioned in my second link above, that having all employees of both companies in one union could be dangerous. Of course that's technically possible now, but unionizing 125k people at once is a challenge, to say the least. Unions have their places, but my experiences have not been good. I think it's more a function of the fact that Big Labor has turned into a Big Business, just like the companies the unions are supposed to be keeping in check. There have been some very shady union-related happenings recently, even in giant national issues like the GM and Chrysler bail outs. Plus the card check bill that still won't go away, which would add a whole other wrinkle to all these issues.
