I have the exact same issue with my 97 5.2.
I would think it was the BCM, but when I plug in my scanner witch uses the same inputs CCD +, CCD - from the PCM I can see speed, temp, and RPM's. And when the cluster is disconnected I lose the readings. This tells me that the signals from the bus are good.. Right??
Also checked all power and grounds to CLuster. All check out.
Even replaced the cluster (used from wrecking yard) does exactly the same thing.
Does anyone know a solution to this? Is it the BCM? I know the VIC is suppose to give similar info that the cluster does but I cant see anywhere on the diagram that ties it into the Cluster.