Vibration

Hey guys,
I'm starting to get some vibration when I take off from a stop that I haven't noticed before. It only happens some times and only until I get up to speed.
I've got a 4.5 in RE lift but the U-Joints feel fine.
Any ideas
Thanks Andy

Pinion is lifting when you accelerate under load...that changes the u-joint angles.

Stock driveshaft or dual-cardan? Either way, I'd look at the rear joint condition first; you may have to physically remove it. If it predates the lift, replace it with a new one anyway.

Are your yoke caps tight? Loose ones will start a vibe as well, and you DON'T want them coming off while driving.