Generally you rotate one axle (or side gear) and not the other and you can get the gear to move into where you can put the pin in. Do one, secure it with the pin, then do the other.
However, I've only worked with 8.8's and Dana axles, so maybe something is different in the 7.5.
One other thing I though of: is your carrier bent or damaged? If the gears got chewed up, that could have been a bit of stress on it.