Can you get your wife to agree to leave
before she sets foot in MEM?
Dallas should be an easy commute. Multiple MEM and IND FDX flights in both directions from DFW. AFW looks like it has one evening flight inbound to MEM 3 nights a week on Tues-Thu. If you get deadhead trips, getting just about anywhere you might need to go should be pretty easy on AA out of DFW.
If you do decide to relocate to Memphis, FedEx does have a move package available for new hires.
Your class may not have a choice, but if you want to have the occasional shot at AFW trips, as others have said, the 757 is probably a good option. That will be your best bet for seniority progression early on. While those senior hotel standby trips encompass a whole week, many of the other AFW trips are short 1-2 day trips that are usually built into a sequence of short trips for a week. So, even though you may not be able to hold the line that contains an AFW trip, you may get lucky occasionally and find one in open time.