2004 Sienna Transmission Shift problems

I have 418,000kms on my Sienna, I developed a transmission shift problem about a week ago. Seems to occur when cold or low speed only. Vehicle does not seem to down shift, if I wait a few seconds or so the transmission will engage. If I try to drive there is no power, van may move very slowly then it kicks in with a thud. I am thinking the shift solenoid needs replacing. Is there any other options?

thanks