2nd gear vibrations

[darks1d3]

I’m familiar with the infamous shudder issue with the 8 speed transmissions but I haven’t seen an instance of a case like mine yet. 
 

My truck vibrates in 2nd gear when over 3k RPM. Every other gear is fine. Shifting is smooth. No deceleration shudder. Any ideas what could be causing the problem? 

 

I took the truck into the dealership. They acknowledged there’s a shake but say it’s normal.