Every location has a package car shop, Phoenix has a trailer and tractor shop. Tempe, Mesa, and Estrella are the other locations. The Airport also has mechanics that work on the tugs and dock equipment, but I am unsure if those are contract or UPS employees.
You can transfer to other locations through the bid language, remaining at your new location for a minimum of 1 year and going to the bottom of the seniority list.
Pay rate seems to be by progression, and I'm seeing a 48 month progression from $18.75 to the current top rate of $35.27 but it's not entirely clear to me that there is a progression so if I find myself at the shop I will try to remember and ask. Pay and benefits are retained when you transfer, just the seniority is affected.
There are pay differentials for shifts starting after 4 pm and midnight ($.20) and midnight to 5:30 ($.25). If your shift runs into another bracket you get the pay differential unless you start before noon in which the whole shift does not receive a pay differential.
Last month or so I remember them saying we were hiring mechanics. UPSjobs. com probably has the most current info.