Just an FYI, if you are planning on using a BEC, this is the best way to wire it (there are multiple ways to wire it, but based on your circumstances this is the best:
With this configuration, the receiver and (diff lock + transmission) shift servos can get power from the ESC's internal BEC (6V) and only the servo can get power from the UBEC at a higher voltage like 7.4V (receiver and shift servos cannot take anything higher than 6v).
As for getting a BEC or not (or which BEC to get), you will need to look at the stall current of your servo (listed below from the Savox website) and the supply current of the BEC.
Savox 1212 Specs:
- Running Current (no load): 120mA @6.0V, 150mA @7.4V
- Stall Current (at locked): 8200mA @6.0V, 10000mA @7.4V
Hobbywing Axe R2 FOC Specs:
- 6V/7.4V Switchable
- BEC Output: Continuous Current of 6A
Ideally, you would want to have the BEC current output to be higher than the stall current of the servo, but you wouldn't be constantly stalling the servo, so 6A would be fine as long as you do not stall the servo. If you think you will be straining the servo fairly often to the point of stalling (or almost stalling), it is recommended that you get a BEC of at least 10A. (Assuming the servo is running on 7.4V; will be 8.2A if running on 6V)
Hope this gives you an overall idea