1996 Chevy Blazer fluid leak

First and formost let me say this vehical has been babied since new. My wife is the only driver. When she drives at highway speed for about 5 miles it blowes trans fluid out of eighther the vent tube or someplace around the exhaust. If she stayes at a speed under 60 miles an hour it doesnt happen at all but if she goes over 60 it will.
The trans shifts very smoothly and does not slip I WILL REPEAT THE TRANS DOES NOT SLIP AND SHIFTS SMOOTHLY. Any help woould be appreciated

thanks
bruce