Is the Auto buggered? If so, it is probably easier to rebuild the box than convert it (although it would probably cost you about $500 more for the rebuild).
If you want to convert for offroad performance, spend the $$ on a set of reduction gears for the transfer case!
There's a guy on here somewhere who did the Auto to Manual Conversion. Cost him around $2,500 and a lot (and I mean a lot) of stuffing around. If you have just rebuilt the Auto though I can't see why you would bother. Maybe better of selling it and buying a Manual?