97 4.3 vibrating

I have a 97 k1500 4/4 5 speed standard with 225000 kms. It satrted vibrating at idle and all speeds a couple days ago. It does not burn oil, fires on all cylinders, new timing chain, new drive belt. The fuel mileage and power have gradually been dropping off. I checked engine and transmission mounts and they are good. Where do I go from there? It's my work truck and I am worried about expensive repairs. Help!

check your harmonic balancer and pulleys for damage or loose bolts.

check your harmonic balancer and pulleys for damage or loose bolts.
Checked pulleys and harmonic balancer and they seem ok. I am not sure how to check the harmonic balancer but I took off the belt and tried towiggle it. It seems tight.

Checked pulleys and harmonic balancer and they seem ok. I am not sure how to check the harmonic balancer but I took off the belt and tried towiggle it. It seems tight.
To check your balancer, look at your timing marks on your balancer, in referance to the location to your rotor, if they don't match, good chance your balancer has "slipped".

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