Okay, so I swapped out my rear u-joints and transmission mount on my TJ. I took it out for a test drive and functionality test. When I would brake hard driving forward, I got some crazy chattering that sounded like it was from my front diff/locker. I knew the front driveshaft needed it's u-joints replaced too so I took it home and disconnected the front driveshaft (in case it was binding) and tried again... Same result. After I got it home i realized the steering stabilizer was oozing, so that got replaced and I tested it again... Same result. I checked tire pressure and they were low, but the same psi on both tires. I filled them and tried again... You guessed it, same result.

Now for some more details:
Newish brakes and rotors (less than 1k miles)
Transmission mount was swapped for a polyurethane mount.
New motor mounts from when I did the body lift (less than a year old)
Only does it when braking hard, no issues under normal braking
Only does it going forward

I have three thoughts about it but I'm open to suggestions. First, the front brakes aren't being applied equally causing slippage in the front diff and making it chatter. Second, the alignment is out of whack and when the front squats it's like a mini death wobble. And third, the new polyurethane transmission mount is transferring more vibrations through than the old worn out rubber mount.

I'm going to get it aligned this week. But other than that, I don't want to keep throwing money at it blindly. Any suggestions on what else to test or what the culprit could be?