Poor Thing

My poor Fintail is complaining like hell cos she's still working after all these years...I've only a month ago finished doing the head, now the key barrel has locked up on me and the diff is whinging like crazy.
The car is triple clunking when she downchanges to a stop, whining during acceleration and grinding (I guess is the best word) whilst decelerating. Had a look at the back end and found it seems to have a bit of freeplay - the wheels still rotate a fair bit when in Park.
Am I pretty safe assuming the diff has had it, or is there something else I could look for first?

Also heard that taking out the diff alone is a pain in the butt, that two of the bolts are barely accessible. Is it true that perhaps it is easier to replace the whole rear end assembly??