i have the factory 10 bolt 3.23 posi in my 86 iroc and it has always spun posi until the other day i was leaving a buddies house and he gave me the "smoke em" signal so i waited til about 25-30mph and hammered it and it started off posi and then in about 10-15 foot into it it kicked into a one legger,after i got down the road a ways i stopped at a dead stop and tried it again and it was posi all the way,my question is ,is this normal since i was rolling 25+mph?or does my fluid need changed?or is my diff just whore out?thanks in advance.

You probaly have the Governor Lock differential. This differential is designed to lock when one wheel spins faster than the other. Under normal use it acts like a standard differential, until one wheel looses traction. It is also designed so that it will not lock up above a certain speed. This is a safety feature to keep an inexperienced driver from loosing controll if they are up to speed on a slick road and one wheel starts to spin. The Governor Lock is designed for light truck use, but many of them were installed in the 3rd gen F body cars.

Governor Lock