Differential question

Ok,
Background:
MP777 SP2 buggy with stock front Diff.
Situation:
I finally got my brushless together 2 days ago and was messing around with it yesterday. Prior to using it yesterday, if I was to raise the buggy off the ground and spin one of the front wheels the other wheel would spin in the opposite direction (as it should).
Yesterday after bashing I noticed that the wheel would spin in the same direction as the opposite side, but they are not locked together (I can hold one tire and spin the other).
I need to order some parts today (preferably before I leave work in 4 hours) and I would rather order anything I might need to fix the diff at the same time.
What are the possibilities in terms of what is wrong with the diff?
Does it just need to be rebuilt? internal gears? outside gear?

I understand that I 100% "this is what is wrong" can probably not be obtained without disecting the diff, but I would rather buy a few extra parts and get them a day sooner, then wait till tonight, forget to order the parts till tomorrow, ect.

Any thoughts? or am I just crazy?
(Please note, answering with opinions and "but you are crazy" is not neccessary as the state of my sanity or lack there of, is not the question, the question is: Asside from my lack of sanity, is there anything wrong with my buggy's diff).
Thanks

[crazyjr]

Unless while working the diff you hear a click or feel it lock, it should be good. If no locking or noise, then the oil might be old and thicker with dirt or metal from the diffs (always a bit of wear, even with oil) I would check everything before just filling with new oil though.

If you plan to order parts anyways, a few diff parts, as spares, wouldn't hurt

thanks for the info