Okay, two possibilities: your vacuum modulator isn't receiving vacuum from the engine (vacuum line off or leaking), or the governor which tells the transmission how fast the car is going, and therefore when to shift, is sticking.

Either or both can be the problem but the first is more common than the second and should be checked first. Tranny fluid coming out of the vacuum modulator on the transmission is a bad sign.

The governor is less likely to be the problem but can be checked while in the car and serviced as needed.

I'm assuming that your fluid level has been verified with the engine running and the trans in park?