You could also have a problem with your turbo. I also have a starwagon and my turbo hits when i'm a little over 65Kph or when my RPM reachers 2K. BTW, did this occur only recently or have you been experiencing this for a while now? Did it occur instantly or gradualy? If gradually, you might have some usual wear and tear problems, or clogs in the inlet or outlet pipes/hoses.

As for the overdrive, try this test: drive along the highway without the O/D on. when you reach aroundn 80-100 kph, hit the O/D button. You should feel a ...surge and feel as if somewhat your Delica is freewheeling or something. With the O/D off, your tach should read around 2.5k-3K at 80kph. With the O/D on at 80kph, it should go down to 2K RPM.

Here's a link where you can get some info on turbo chargers and how to perform basic diagnostics: http://turbotech.com.au/trouble2.htm

Good luck.