How do I fix rumbling @ 40mph

Question

I have a 2014 chevy 1500 z71 with a 7.5 inch rough country lift with 35s and about a year ago I had the transmission and torque converter rebuilt.before it was rebuilt the truck would fell like it was rumbling/vibrate going down the road or running over the rumble strips on the shoulder mainly from 45 to 60 mph. After the transmission and torque converter was rebuilt the issues went away.
Well within the last week the truck has been starting to feel like it’s starting to vibrate/rumble again but this time it’s between 30-48 mph and it’s only doing it when I first start driving it in morning and after sitting all night.about 2-8 miles the rumble/vibration seems to go away for the day and will only do it the next day. There’s no code or check engine light coming on .
also the tires have been rotated and have been checked for alignment and balanced . I have no ideas on what it could be if not the transmission or torque converter. Could you please direct me in a direction on what it could be causing the issue

Doc · Accepted Answer

The extreme angles of the rear drive shaft u joints as a result of the lift kit make me suspect them as the culprit.

MountainManJoe · Answer

Posted by: @shawnz71 7.5 inch rough country lift with 35s there's your problem

Justin Shepherd · Answer

Posted by: @shawnz71

I have a 2014 chevy 1500 z71 with a 7.5 inch rough country lift with 35s

If that lift isn't from the factory (Chevrolet makes a High Country trim package that's slightly lifted) then @Doc is probably correct. You're destroying your drivetrain with a 7.5 inch aftermarket lift. And those wheels. Monster trucks drive on dirt, not paved roads. Dirt and tires slip. Asphalt and tires do not. Unless you drive on unpaved, really uneven surfaces for 90%+ of your driving, there's no reason to lift a truck like that. It's dangerous.

My Ranger has the off-road package that has 33" tires from the factory. Get a truck with a slight lift from the factory if that is your thing and leave it alone. Axles are supposed to be relatively straight, not bowed down 20 degrees.