There is also a lot of slop in the front IRS sub-frame bolts. They were having a hard time on the assembly line installing the IRS due to the tolerance spread and asked if they could use smaller bolt to make lineing up the holes easier. Some cars have 14 mm bolts but most have 13 mm. When you apply reverse torque loads the suspension may rock a bit in the holes cuasing the clunk. My car does it some time but it seems less as I put more miles on it. Some guys have sucessufully eliminated the cluck when they upgraded their rear suspensions. We shimmed the pinion to .008" backlash which is on the tight side (factory spec is .008 - .012) so hopefully that will help.
If that don't do it then it's all in the T-56
If that don't do it then it's all in the T-56