for the 3.42 you will want a GU6 axle, you will find them listed mostly for the yukons or escalade but it's the same fitment, if you poke around ebay you can get them for around $1300 +/- with free shipping, a local higher end dismantler may have them as well.
other than that you will need to check local prices for the swap or if you have the tools you can DIY
again I could be wrong on the cost I was quoted a large amount to change over the pinions on my Silverado and after a little digging figured out it would be cheaper to just swap axles, mine has the 3.08 and I want 3.42 probably get it done this summer hopefully the 3.08 is kind of a dog to me after a lift and slightly bigger tires it doesn't want to stay in high gear on the freeway much my average speed is 85 and keeping it there it's downshifting often.
other than that you will need to check local prices for the swap or if you have the tools you can DIY
again I could be wrong on the cost I was quoted a large amount to change over the pinions on my Silverado and after a little digging figured out it would be cheaper to just swap axles, mine has the 3.08 and I want 3.42 probably get it done this summer hopefully the 3.08 is kind of a dog to me after a lift and slightly bigger tires it doesn't want to stay in high gear on the freeway much my average speed is 85 and keeping it there it's downshifting often.