Vibration under my feet

hey everyone, I got a 03 classic with a 12.7 Detroit, I noticed while pulling a hill, when you get to the top and let off the pedal I get a vibration under my feet. I had the shop look at the driveshaft but he says everything's all good. I tried explaining the problem but he doesn't know. It does it all the time, could it be a week injector? He looked at the motor mounts and nothing. I just can't think what the prob could be, it's been aligned all new tires. Any help will be appreciated.

Thank you

 

I'll bring that up thanks guys.
I had all new u joints put in when I had it alighined, this was in early July. This problem started this week. I'll have everything looked at again, I appreciate it

 

I can sometimes feel the vibration in the pedal and can so times hear it through the transmission. Thanks for the input. The truck also has a hop in the front end it bounces up and down while bobtail, there was another truck that did that and it had s bad injector, that's what the mechanic was leaning towords. But I'm more agreeing with you.

 

Hey I wanna say thanks, mechanic found play in a u joint and when I mentioned the bag height he scratched his head sure enough, it was high. Appreciate the hell