Clunk from front of vehicle

It ran great when I left work. Came home and parked in driveway. A slight slope. Went to bed and when I woke up, she started right up, but as I started to reverse, it felt like it was binding. Like I was hitting something. Almost like the differential broke. But the noise is coming from the front not the back. And when I put the transmission in reverse, I hear a clunk. Never in drive, only reverse. I’m stumped. Mechanic said transfer case is fine. Is this the u-joint? I’m so lost and need help!