My advice is don't bother. The Losi RTR diff is a leaky thing and another seal won't change this. I recommend getting a decent gear diff. The TLR race diff for the 22 4.0 is what I use in my 22 SCT 3.0 and it is fantastic. Some have used the Serpent gear diff to good effect. They are both expensive but will work and don't leak.
TLR diff = TLR332067
Serpent diff = 500192