Bad vibration at start, when giving throttle
 

I started my truck this morning and it ran fine, I left it idling with the heater on because it was cold and when I came back and left, it had a very bad shake/shudder.

at idle, it's not noticeable. The engine seems to be running smooth and fires right up as it normally does but when I give it any gas it shakes, a LOT.
I looked under the hood and all of the stuff run by the belt seems fine and it seems to be coming from.the back of the motor. Any insights? Could this be related to my flex plate?

Best guess is that its time for plugs wires and a cap if its been a while. Slightly compromised plugs and wires can cause a plug to foul. Often it clears up by itself. It just might be an indication that you should do some routine maintenance. Sometimes this kind of thing can just be a passing glitch. Yesterday was our first snow and low temps. It was fairly ugly on the roads for NJ. Lots of very slow driving and wet. I got a misfire code and a service light. I cleared the code and it didn't return.

It's not a misfire, engine would probably throw a misfire code and it would have to be 2 cylinders at least to shake the entire dash like that. Sometgings off balance, really off balance. I'm pretty sure its transmission related it's been having a ting jingle ling and tack tack tack coming from the torque converter area for awhile now. Does it in park and neutral but stops when in gear. I think I drove it 10k miles like that and 20 since I replaced the transmission. I've been meaning to get in there and check it out and fix it, but pulling the transmission to check out the torque converter and flex plate is a lot of work. I'm in it now though and loose bolts from flex plate to torque converter. Holes are ovaled a little but other than that i dont see anything, though i havent fully dropped the transmission because I haven't removed the crossmember. I was hoping to just slide it back enough to pull the flex plate and immediately see an issue.

Tighten the bolts, and see if your noise/vibration goes away.

The noise certainly did go away but I'm left with the vibration. I'm starting to think *misfire* or something more serious.

Last time the truck had a tuneup? Have you fixed the plenum yet?

The last work i did was replacing the heads, cat, and water pump. I did a plenum gasket then, about 15,000 miles ago. I just replaced the coil, cap & rotor before taking the videos, sounds like rod knock in the vid but its really just the dash shaking, there is not really a knock at all or any abnormal engine sound aside from some slight lifter tick.
https://youtu.be/HygOqFjQVyc
https://youtu.be/gHkJ3ia7vfk

I would think if the holes on your flex plate are ovaled, that that's not a good thing. It could cracked too and you might not know it as they can be hard to see. If that is indeed the problem, I wouldn't drive the truck till its fixed right. Dangerous stuff and probably not doing your engine any good.

I just replaced the flex plate and torque converter. Sure was easy, not. But actually it was easy compared to actually replacing the transmission, that crossmember is a pita and requires a BFH to reinstall, and I left it in. I found just disconnecting the trans and pulling it back and to the side, then removing the flex plate, you can remove the torque converter without dropping the pipes. I mangled the crank sensor, so after reinstall got a no Start but since I know already about the crank sensor I replaced that first and it fired right up, with the same issue minus the jingle from the flex plate area.