91 grand caravan awd auto park sift problem

please help my problem
my grand caravan se 91 auto awd 3.3l doesen't stop moving after park the car and sift park si have to use hand break to hold car stop.
also over 50m/h transmission slipping if any one has any idea or gess?

Your shifter cable could be misadjusted, or broken.
It is likely that instead of the transmission actually "slipping", that you are not actually in "drive" when the indicator reads "D" and may be in "2" or "3"

Or, the park pin could be broken and your transmission could have seen better days and needs to be replaced.... a much more expensive option.

Personally, I like the first option I mentioned.

Tom