Since we are on the subject of simple but effective tools, here is my diff pinion locking tool:
it is made from a piece of 30x8mm Mild Steel about 500mm long. The holes are 10mm diameter and 50mm between centres. The important bit is to grind out one side to allow acces to the diff pinion nut.
In use you need to undo the propshaft from the diff nose, and bolt this on accross two of the holes. The ground out piece means you should still be able to get a socket onto the pinion nut, and the other end of the tool will brace against the chassis rail.
It may seem obvious, but it makes doing the pinion oil seal a lot easier!
Enjoy
Mark