Well got the fuel tank off and the servo out. Checked servo, same as the other one. I connected the electric motor of the servo directly to a 5 volt battery, and no go, so seems like the little motor died and not the electronics. So much for the trouble shooting I could do here.
Had a really hard time getting the side covers of the fuel tank. Good thing I didn't glue them as suggested in the manual!
Stuck the spare servo in there, and hooked it all up to the receiver and then switched diff lock on and off a couple of times. This servo seems to be behaving. (Actually should have used receiver/transmitter the first time too.)
Diff unlocked
Locked
Stuck it all in the fuel tank and then locked/unlocked about 40 - 50 times ...
Very tight fit though.
Unlocked
Locked - The nut of the cable holder is almost touching the sides. Thinking of adding 4 or even 5 washers instead of 3, but will have to file some of the ridge to make sure the servo fits then.
I dinged the tank taking it out, so will need a bit of sanding and painting again ...