While I have heaps of experience with engines, a crawler's undercarriage is a something new to me. 

I started by pulling off the right track which had "toe out" due to what I suspected was a bent track pivot shaft.  On getting the track and frame off my theory was confirmed. 

The right track frame pivot shaft was bent back by about 1/2 an inch at the outside edge causing the right track to "toe out" by about 2 inches at the front.  The worn track bushes are obvious in the third photo and there is some wear on the main drive sprockets.  One link has been removed from each track at some stage as well to take up the wear.

A few weeks later I pulled the other side off and removed the bent pivot shaft. I was planning on getting a new one made but was quoted up to $500 for that job and decided a second hand one may be more within budget.  The first place I tried (Horstman Tractors in Salisbury, SA) had one on a 4U that had been wrecked for parts and after removing the track frames I found the shaft to be almost like new.  I paid $250 for it which to date was my most expensive single purchase for the tractor but it's cheaper than a new one and cheaper than anyone else I rang as well.

I also got some decent replacement sprockets from Allan Horstman and left my tracks with him to get the pins and bushes done.  The old track tensioning bolts had really worn and rusted threads so my father-in-law offered to have a go at making some new ones and they turned out really well.  Another problem was the roller bushes were nearly all worn so I knocked the old ones out and made up new ones out of bronze bush stock.