It sounds like you have 2 separate issues. 1stly, your rear diff is worn & has too much backlash, which is why the "slackness" gets taken up when you accellerate. 2ndly the clunk is most likely your panhard rod- either the bushes or where it mounts onto the chassis. Check that the bolt eyelet on the chassis is round & not egg shaped & torque the CRUD outa that bolt. I mean SWING off it!!! Also check your radius arm bushes. They'll cause this as will worn tie rod ends.