Could be a number of things wrong.

Here's a few questions to try to narrow it down:

1. Have you got a "check engine" light coming on whilst driving?
2. Does the odometer still work?
3. By VS most (but not all) utes had a speed alert. If yours has it, set the speed alert to something real low (say 45km/h - I can't remember the lowest they'll go) and exceed that speed (use a GPS) - does the speed alert beep & light up the LED?

The speedo signal goes:

1. Speed sensor (gearbox) >
2. ECM (computer) >
(splits in 2):
3. Instrument cluster (speedo).
3. Speed alert

If the speed sensor is not working, it will normally throw a fault code.
If you don't have a fault code, then it generally means the ECM is seeing a speed signal. Also, without a speed signal, manuals will tend to stall when slowing down.

So this is why I suggest checking the speed alert - if it works, it narrows the fault down to the cluster.
If it doesn't, then the fault is in the ECM or speed sensor.

Check those 3 things I listed & see how you go.