The most likely cause for failing to come back to idle (when the throttle is closed) is that the engine is running on its own lube oil. The oil can come from the positive crankcase ventilation, the turbo seals or the valve seals. I vote for the turbo seals. Perhaps the rebuilder did not rebuild the turbo.
Paul, thanks for your reply. Would this be possible without any visible smoking or high oil consumption? I haven't driven this vehicle a lot, maybe 1,000 miles- but didn't notice any significant oil consumption or visible tailpipe smoke during that time. The engine also has virtually no blowby visible if I remove the oil filler cap during idle.
Is there some way to test your idea without pulling the turbo off? Maybe I should pull the intake manifold hose, such that the intake is open air at idle, without the turbo pushing into it?
Also, it doesn't rev way up out of control like it's "running away", more like it's just idling real high. I don't think it will maintain rpms over 2,500 on it's own, if I were to push the clutch in at 3,000rpm it would drop to 2,000-2,500 rpm.