The cheapest way to lock your diff is to weld it up;). But this abviously is not legal and can be scary in the wet.
If you just use it for play and live close to your favorite play ground a welded up diff is great. It only takes a few hours to swap the open diff back when you finish playing. But you have to be keen.
BTW I too am looking for a Mcnamara diff lock for my old truck. let me know if you find 2 of them.
cheers
Will.

