Tourque converter lock-up slipping

when i take my 98 blazer out on the highway if i get the truck to enter overdrive i get a check engine light, i had it scanned had it came up with " trans conponent slip" my parents took it on a road trip over the summer(35 degrees celcis 4 people with luggage doing 120 kmh in a head wind) long story short the tranny hver heated and spewed fluid all up the backside of my poor truck(not to mention they left me to clean it) now whenever i go on the highway the get a check engine light if i go over 80kmh, also the check engine light comes on is i slow down to 70kmh from any speed higher, has anyone had the same problem if so do i need to replace the transmission
thanks