rear diff making noise

[abmobil]

I was just wondering if there was any other way to tell if the rear diff was going out other than whining.
This is not about a Titan but about my 93 harbody regular cab 4 cyl work truck.
Under light acceleration there is a light tapping or clunking coming from the rear of the truck.
I put the truck on jack stands and let it run in 3rd gear and it definently sounded like it was in the rear diff, but when I put it in neatral and spin the tires by hand they both spin freely and it makes no noise.

I also tried spinning the driveshaft by hand and everything turned smooth.
I bought this truck about a month ago with 98k on it and changed the diff oil to amsoil severe gear 75w90 the next day.
im hoping its maybe Ujoints but the shaft feels very tight.

I will admit that I did a couple of burnouts holding the brake in a wide open space where no one was around just to take some pics.
Any fedeback would be great.
Thanks!