12 bolt/autocross problems?

[white79bu]

The past few weeks I have been autocrossing my 79 Malibu. The car has been driven very little the past few years due to life.

The car has a 12 bolt with a Brute Strength posi that has never had an issue. But I have noticed that the car seems to be spinning one tire at times and it's killing my autocross times. But the car will spin both tires in a straight line. I was kind of thinking the diff may need alittle fluid. I haven't touched the rearend for probably 10 years.

I did install a DSE rear sway bar that is very stiff but I don't think that would have anything to do with the rearend.