Cobalt is the one who put 3.90s in his Eaton. And it
was pretty expensive. Parts for these rear ends aren't cheap. Might I suggest you hit the junkyards in search of a Dana 60? You could find them with 3.54s, and if you really want 3.73s you can buy those for a D60, too. (But that ratio didn't come in D60s in these trucks.)
The Eaton is a far tougher rear end, and might be worth a rebuild if you need it, but the Dana is no slouch. Good luck.
