should there be any oil / grease in the diff's on the RXT-1 ???

Oil in the diff case (spyder gears)  I personally like 7 in the front 10 in the center and 3 in the rear, the ring and pinion gears should have grease on them.

What kind of grease goes on the ring and pinion gears? I changed out my differential gear oil, but failed to buy the grease for the ring and pinion. So what goes on them?

I have used general purpose grease in mine with no problems at all.

so did I - just plain old grease from the auto store - and it does not need to be packed full - just put a few pumps of grease in, enough to coat the teeth on both the ring and pinion gears

you should also take a look at shimming the pinion for good ring and pinion mesh if you have not already done so

Forgot to mention about the shimming! The diff gears are already really good and will last quite awhile but with good shimming you wont have any problems from them unless of course if something else became out-of-whack first.