*Depends what kind of differential you have.
Standard TracLoc would require a synthetic with friction modifier.
If you have the Track Pack, this includes Torsen helical-type differential, yes?
If that is the case, you should check Torsen's recommendation. I know I have an Eaton TruTrac (Eaton's helical type) and they strongly recommend a mineral-based gear oil (non-synthetic); no friction modifier required because there are no clutch packs in a helical diff. Apparently, synthetic oil does not dissipate the heat very well to the housing and causes premature failure of the helical gears and teeth under heavy use at the track. Consult your differential manufacturer!
If you put a performance cover on your rear end, you certainly want to make sure heat is dissipated from the fluid to the carrier and fanned out through your cover as planned!
If the axle vent is leaking large amounts (a little bit is normal under duress), you probably need new seals. It sounds like there is a lot of fluid expansion happening in your carrier... maybe due to the expansion properties of the viscosity of oil you are using.