I would start here, (while you are looking, check the vacuum line from the intake to the modulator valve, its just behind the trans pan, the rubber sometimes swells or the steel tube gets rubbed through and a small vacuum leak in the hose or tube can cause havoc in the shifting of a C-6. Also make sure no fluid leaks out of the modulator or hose when you disconnect it. (I've seen new ones fail right out of the box.)Id start by checking your vac at idle, you said it was around 17", then rev it to around 2000 in neutral, the vacuum should climb around 1-2" (It will drop as you open the throttle, and then recover as Tiha said.)
(The engine is more efficient at 2000, the vac should be better)
If it goes down but is running smoothly, you likely have restricted exhaust, drop the pipes at the manifolds (I they will come off without breaking all the studs) and check again. If it comes up now, the exhaust is plugged.
If it runs bad at 2000 RPM with low vacuum, chase the engine performance first.