Transmission issue - Alphard 20 series 3.5L 4WD

[MarkF01]

Hi All,

I've 20 series 3.5L V6 Alphard. I've got a what my normal garage think is a transmission issue, possibly with the transport box. They've recommended finding a transmission specialist in Birmingham to help diagnose.
The car wouldn't pull off in drive, juddered heavily and then appears stuck in 4WD mode. It now drives but there is a 'dull belt' sounding noise at slow speeds. Im based in Birmingham and have already tried the following places; Driver Transmissions, Neil Smith Automatic Transmissions and Toyota main dealer.
 

Any suggestions would be greatly appreciated please.
Thanks, Mark