Is it brake fluid, or some other fluid that accumulated in the brake section of the rear axle housing ? Mostly it leaks from the housing through the big round hole through which the hand brake pullrod goes. If it seeps from there, and youre sure its brake fluid, you would need to replace the slave cylinder. And when you've got the stuff open, replace the oil seals as well as they'll leak once youve stretched the 30 year old seals when wiggling the halfshaft back into the differential.
Also there is a bearing beside the brakes, i allways replace that too because the life is limited due to brake heat. Its better to replace that ball bearing, the oil seal and the gasket that seals the brake drum on the shaft.