Welcome to the forum!
I don't know about the ECU from back in those days, so if there's a fault that has triggered it into limiting the rpm (like the new ones do) I couldn't say - you might check if your engine warning light is on.
If it's not the ECU, there could be a few things to check.
Insufficient air flow will inhibit the engine's ability to combust the fuel. You will see lots of smoke and may even be able to smell unburnt diesel in the exhaust pipe (once you have turned the engine off). Check the air cleaner, and the hoses going to it (including the snorkel and the joins through the guard).
I don't know if your engine has an EGR valve, but maybe that's stuck - it should close at high throttle, and if it doesn't there's a chance it will over-inhibit the combustion process.
You might have a restriction in the exhaust but I'd be looking at the air intake first.
There might be an issue with the injectors, or fuel rail pressure - but let's take a look at the items that don't cost anything to check first!