shifting weird

[evil_motors]

okay finally picked up my astro from my parents house today and drove it home.. here is what it does.. if i am in overdrive the tranny will never shift until i put it in drive or 2... i never got up past 50 to see if it would shift again but that is what i have to do.. then i had to shift it back into overdrive when i came to a stop or my brakes would lock up .. it was weird so it probably doesnt have anything to do with it.. so my question is what would make it not shift in overdrive but shift when i put it in drive or 2.. and it doesnt do it gently either.. it chunks into the next gear.. i have heard it is some vaccum thing or even the sensor where the speedo cable goes to the tranny.. so if anyone knows please help since i cant get on astrosafari.com


thanks]
Mike