Squeal/Squeak in reverse only

So I haven't found much info on this issue I'm having. Only in reverse my truck makes a chirping squeal/squeak sound. I thought u joints like everyone else. So anyways I take my truck in to be sure it's u joints so I can buy some. But mechanic calls me out in shop to under truck n see for myself where the noise is coming from. He greased u joints generously but I had done them like I usually do during oil changes. Drive shaft was still tight, so u joints good. But with a stethoscope the squeal seemed to be more pronounced in the rear drive shaft just between the rear axle n carrier bearing. I thought maybe u joint goin bad and noise traveling down rear drive shaft but u joints are good. Anybody had this issue? Any input? Advice? Etc. notes-
No vibration on steering wheel or pedal
I have a toyotech lift kit w adjustable height up front(only mod)
Noise started out of the blue one day when reversing(vehicle hadn't been driven for 2 & 1/2 weeks)
3.4 v6 dbl cab TRD but no rear locker)

 

Sound the same with the clutch in as out?

 

You need to take the drive shaft completely off to actually test the u-joints. You won't be able to tell if they're bad by just shaking it while it's attached.

 

I have an automatic.

Mechanic had rear end up, tires were in air. Everything felt firm n tight.

 

Anybody got an idea?