These thing leak all the time, the lighter the oil the more the leak.
If youve stripped out the casing youll just have to replace the 2.6 x 8 screws with 3x8 screws!(local hobby shop)
Make sure though that the gasket and housings are all aligned right.
Dissasemble the diff, clean the diff housing where the screws go in,
get a 2-part type epoxy glue and just put a little on the thread where the screws go in. give it a couple of hours and re-assemble.
Thats as quick as it gets!
The o-rings are behind the out drives and have nothing to do with the leak!
dont panic dude......thats nitro
If worst comes to worst a complete new pulley/diff set is only about $8.00 usd