2000 Sienna acceleration problem

It sounds like you have throttle body problem. On a 2000 Sienna, the accelerator is not physically connected to the throttle. The pedal position tells the computer how much gas the driver is calling for and then, depending on the situational condition the computer has detected (I'm unsure of all the variables but it includes the vehicle speed, speed of each individual wheel, etc), the computer tells the throttle motor to adjust the throttle to change the speed of the van. There is a throttle position sensor in the throttle body which tells the computer the position of the throttle and this position sensor or the motor system which opens the throttle may be malfunctioning. This could be because there is buildup inside the throttle body which prohibits the throttle from opening/closing. If you can do your own maintenance I would clean your throttle body first and see if that helps. Then I would replace the throttle body if that doesn't fix your problem. If you find a lot of buildup inside the throttle body, that may be a good sign you've found the root cause. There are many videos out there that will instruct you on how to perform these maintenance steps.