Use Car-part.com to find out what the entire set up (used) is going for. Type in your zip and you'll get salvage yard results from your area first, then beyond, then BFE.

An axle with all the goodies isn't going to be cheap IMO. But have a look.

 

Craigslist!....I got my 8.8 with LS and 4.88's out of a 98 f150 off of Craigslist for $275 last year, then I sold mine for $150...car-parts is a good site as well, though I never get lucky on it

 

Right, on car-part, your usually going to pay the same salvage yard price in most cases. Still cheaper than new and being that the OP is looking for the best option, like,- the 9.75 LS w/drive shaft. The larger gear not only handles more torque, it produces more. The 3:55 ratio is more common, -3:73's are out there tho. I pulled a car hauler for years and 3:55's worked alright for me. 3:73's would have been better most likely. 4:10's for/if your running 33's -35's I expect. That's for keeping the truck versatile and as economical as you can.

Like you said, Craigs List. You can get things for a steel on that site. Killer deals! I have lol. Can be a little scary. One deal that came around was on a nice Miller welder. I went to the address, was invited in and sat on the sellers couch between two pit bulls. I think it was the dogs couch iduno. Thinking wow, what have I gotten myself into here lol. So, just be careful.

 

JBrew that sounds like something straight out of the movie Deliverance.