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All things pertaining to wireless and RF links
By stevech
cannot do servo-type PWM through an XBee alone. Not fast enough.
Best to do this with an RC receiver and transmitter.

ELse you have a lot of work to DIY the equivalent with your programmed microprocessor on each end, and transmitter/receivers.
By Valen
Write a Redboard program that reads the serial port with the Xbee continuously and make it interpret a custom command that you send to it containing the desired motor strengths. Then update the PWM motor values accordingly with the right Redboard function.

You'll have to send the command some way manually by typing on the pc into a serial terminal program sending to the XBee local.

There are enough remote command programs made for arduino from which you can take inspiration or examples. The tricky part is making sure the command interpretation ignores faulty received commands.