Looks like we are on a similar course agaIn with this project.
I don't know how you were testing the RFM22B modules, but I have them hooked up to the 3.3v of the Arduino, and I found I need a 4700mFd cap on the 3v3 supply line to take the pulse when it transmits at full power.
I also want to monitor the SSR reading every second or so, at both ends, and display it on the LCD with a warning light if either drops below a certain level.
I am thinking of having the robot send its housekeeping data every second, so as to monitor the comms, and hold back when receiving " driving" data from the remote.
I am going to add a LED flash ( can't use pin13 as its SPI ) and take a walk with the one arduino on a battery tomorrow, I will try it with the 170mm whips both ends.
Have you got the RX and TX tied to the GPIO pins ?
I will post the range results tomorrow.