does anybody know a rough guess on how much it would cost to swap a 96 6cyl 2wd chevy truck to a diesel 6.6 4wd?

I don't want to deter you but it would probably be easier to just by a duramax, 4 wheel drive truck. That would be tons of work! Swapping both axles(if you want 3/4 ton front and rear), engine swap, different trans.(manual would be easier, allison would be even more work), and adding the transfer case.

It would be doable but a lot of work.

There is a member here who swapped a Dmax in an older body style Chevy Crew-Cab. I think it was a late 70's model year truck. He put a manual tranny behind it, and the truck was already 4 wheel drive. I can't remember his user name, but I'm sure someone will chime in who does.
His setup would def. be worth taking a look at, if you decide to do this.

do it you would have the only one on the block and that would be a fast truck find a wrecked truck and you can get most of the parts from it