Yaris - Wobble/Vibration... Help?

[Eaks]

Good day,

I have a 2007 Toyota Yaris 4 door HB.

When driving highway speeds 80-110KM/hr (50-70MPH), the car wobbles/vibrates, only on acceleration (regardless if aggressive or subtle). Pretty sure it's not a warped rotor or wheel balance because it's not the steering wheel that wobbles.

I took it to a mechanic, he suggests it's the transmission and for me to take it to a trans shop. Before doing so, I wanted to hear if there were any other suggestions or concur that ya.... probably the transmission? If so, what in the transmission could be broken? Worth fixing?

When around 90km/hr i'll drop it down to 3 gear (out of overdrive) and there will still be a wobble... maybe not as bad.. hard to tell.

Thank you!