Bad...something is bad, need some Cat help.

Been perusing many resources to find my "shake". Came across some stuff, but not 100% sure, so let me start with the history. Truck shakes at 60-70 mph, but after chasing all the suspension "ghosts", I realized it happens at 1200-1350 rpm, which is the above mentioned speeds in 13th. So, before we get to looking at what it MIGHT be, let me start with what work has been recently done:
Entire front end rebuilt (tie rods, connecting rod, kingpins, ball joints and spring hanger bushings) all within last 65,000.
Transmission, clutch, rear main, rear motor mounts, all u-joints and pilot bearing about 15,000.
Rear leaf spring pivot blocks 18,000.
Drive axle shocks about 80,000, fronts when the shaking started a couple days ago.
Cab shocks, cab mount bushings and airbags about 10,000.
So, up until about 4 days ago, the 2000 Pete with a 2005 Cat C15 (pre acert, single turbo) was riding like a Cadillac. Then the shaking just started. I've also since looking at the motor began to hearing a "knock" when I rev it to the mentioned rpm, but it is an inconsistent knock, unlike when a valve is tapping. Motor idles fine, no shake or shimmy. And once it gets around 1400 rpm, everything just goes away, including the knock. I've read it could be an injector. Any way to remedy this besides replacing them? I've ran that Hot Shot cleaner from Loves through the fuel, but not the oil yet...$75 a bottle is rather steep, plus they were out when I bought the Fuel stuff. Is it worth a shot to try it before plinking down another $2500 to replace the injectors?

 

Not tires, pretty sure. The vibration is there when at a stop and rev to rpm range. Pretty sure its motor. Pulled the valve covers and checked things out...can't pinpoint the exact spot the "knock" is coming from. Checked valve lashings on the ones that were off the lobes, all well within spec. Used depth gauge on injectors, all dead on 1.50". Cam lobes? Not visible... But rocker bearings are getting plenty of oil over them and when running appear to be rolling fine. No broken springs, nothing appears to be wrong...traced the injector wiring all around,no breaks and has pliable insulation, all connections are tight...now I'm heavily leaning towards injectors. But, what am I looking for on the damper? Its there, its round and its Cat yeller...

 

I used a "hillbilly stethoscope"! A bar held up to my ear. Noise is prevalent between #1 and #6...but loudest between #3 & #4 along the valve covers, equal on driver/passenger. Checked the balancer, no signs of deformity.
The truck sat and idled with the Hot Shot fuel additive for 18 hours while loading (##### Dial in Edwards ville, IL). Loaded weight 79,450. Knock is less, and vibration is gone. Decided against the oil additive, as I am real close to due for an oil change and thinning the oil may cause other problems. Added a full gallon of Lucas injector cleaner to both full tanks to see what happens.