drive train clunk

I have a question for you guy's . my brother has a 2000 rcsb chevy that he just bought it 2 weeks ago .3 days after i stared making a loud clunck when it would shift so i triedremoving the battery cable to reset the pcm and i worked . well 1 week and a half later and it's back . i checked the u joints motor and trans mounts . every thing's good . it's a very loud clunk at 1-2 shift and a little softer 2-3 shift maybe someone has had the same problem before . thanx for the help

Is the tranny completely stock? If not, what upgrades?

is it lowered, could be the pinion angle.

it's completly stock . it's a 4.8

Try cleaning and lubing the driveshaft yoke. That'a a common source of clunks.