There are a couple good threads on this forum and Clubfrontier.org that go over the recovery plan when the trans goes bad:
1. If you have warranty, let Nissan deal with it. Since you have an 05, I assume you're in the same boat as me, out of the extended mileage window and SOL. On to #2.
2. Find a wrecked trans and get it put in. Replace the rad. If you look, you could probably find a wrecked pathy or frontier (i'm thinking insurance writeoff auction or wrecker) to be a donor. Rads can be had from Rockauto for LT $200.
3. Rebuild trans, replace rad - Below I included a link to a thread from some time ago, the guy claimed to have rebuilt the trans for somewhere in the neighborhood of $2000.
http://www.thenissanpath.com/viewtopic.php?t=1987
Let us know what you do. I've bypassed my rad, and it seems ok, but haven't discounted the possibility of a trans replacement someday as I plan to drive the wheels off of her, and I tow big (BIG).
Good luck.