Do you have power-shift/spin-out rims on the rear wheels. Clunking is very common if all the eccentrics on all 4 rails are not tight. Check them first. They may be rusted if not used in a while. Spray some PB Blaster in them before trying to tighten/loosen. They can be disassembled by removing the snap rings allowing you to really clean them up and put some never-seize on them. Some have suggested throwing really bad ones into a hot fire to free them up. I've done a couple by heating with a torch.
Also some run-out would not be uncommon for spin out rims.
If this is not the issue and you think you have a bad bearing in the final drive, I'd just find a good whole final drive unit from a parts tractor or salvage yard and swap the whole thing rather than tear apart the final drive. Mike