'03 TJ Trans/T-Case Vibration during Acceleration

I have a 2003 Jeep TJ 4.0L V6 5-speed manual. Roughly 70k miles, heavily off-roaded.

Modifications:

• 4.5" long arm Rough Country lift
• 33" tires
• SYE on rear driveshaft
• Tom Woods custom front driveshaft (currently removed)
• Stock Dana 30 Front / Dana 35 Rear with 4.10 gears, and an Aussie/lunchbox locker

I've been chasing a throbbing/clicking noise for a while. The vibration only occurs under load (light / moderate / high acceleration) and seems to be in either the transmission or the transfer case. It is slower than engine speed and higher than tire speed, and roughly triples in frequency (occurrence of noise, not pitch) when the vehicle is in 4 low. I only hear the clicking vibration in 1-5th gear, it goes away while coasting or engine braking. The issue is present at all driving speeds.

I tried engine braking in reverse to see if it was one side of a gear in the trans/t-case, and could not hear the noise (although that may be due to the loud gear whine). Also is not present when the clutch is pressed in, only when fully engaged so I don't think it's the throw-out bearing.

I've replaced both front wheel bearings, and every u-joint on the vehicle other than the two in the double-cardan joint on the rear axle which I cannot move by hand (and do not think they are bad?).

I have ruled out the front axle and driveshaft by removing the front driveshaft and the noise is still there, and the sound did not change. I do not think it's the ring gear or pinion, since the noise increases in speed when in 4LO and wheel speed stays the same.

Looking for any troubleshooting tips or ideas of what to look at next. Thanks in advance!

I didn't see any other posts describing this particular issue, if they exist please direct me to them!