Isuzu drivelines as a rule generally clunk, thanks to the heavy-truck lineage that's built-in. If it's shifting fine and you're getting metallic clanks as well, then you may want to take a hard look at your u-joints. Especially considering the mileage.

Our IronMan just started doing this (120K+ miles) and there's a lot of play in the u-joints, so I'll replace them this coming weekend which will return us to just the clunks.