Vibration while pressing gas

So driving my truck yesterday 98 K1500 5.7 Extended cab factory suspension with 215K

I started to notice that under acceleration or just cruising while pressing the gas pedal I would start to get a vibration through the cab. As soon as I would let off the gas the vibration would go away. I press the gas again and the vibration comes right back. I thought maybe a motor mount. so I threw the truck in neutral while rolling at 40 and pressed the gas and no vibration. so I think that rules the engine out.

Thinking possible u joints? or something transmission related. any help or direction on where to look would be great thanks.

and if it is the U joints I will probably go with Moog but who knows what part number I would need as when looking up at a parts store they list about 15 different u joints

Thanks for any help

I'd guess u-joints.....

It does sound like U joints

U-joints or you somehow threw your driveshaft out of balance.

So its not u joints. Replaced them this weekend. Luckly the prior owner replaced them at some point prior so I didn't have to melt them out. but driving to work this morning the vibration is still there.

One thing I am noticing is that it gets worse with speed now. as in around 40 -50 MPH it is there but not that bad. Around 70-75 its all you can notice is going on till you let off the gas.

Could it be something as simple as a wheel being slightly out of balance? and the vibration just gets worse with speed. but I guess that doesn't really explain why the vibration goes away when I release the gas pedal.

I love the truck but this issue is getting annoying.

I would also check the harmonic balancer. It might be seperating

could a pinion yoke cause it? or driveshaft weight?

Pick the cheapest thing to replace next. Process of elimination. Sometimes you just gotta do that.

Trans mount

Okay so update on this. Ended up pulling the driveshaft back off and checking the front u joint again and appears either I got a faulty u joint or installed it wrong. Either way it got replaced. and now new symptoms sort of.

So what is happening now. I can accelerate just fine with no vibration at all. There is how ever a vibration now when I release the gas. and this vibration only occurs for maybe 2-3 seconds long after releasing the gas. Also only occurs at speeds greater than 45-50 MPH.

so thoughts on this issue?