-Is the steering shaft extension a must have with the lift kit installed on the truck?
If I remember correctly, if you buy an entire kit, it should contain everything you need to complete the installation. This would include bumper brackets, fan shroud relocation brackets and anything else you need. I've also seen kits which included shifter extensions. I am not familiar with either of the kits you mentioned and I usually prefer to put together my own kits because I can do it cheaper.
As far as your steering, you may or may not need a steering column extension. I usually look at the shaft. If it has a u-joint at the steering box or rack & pinion end, and a rag joint at the other end (near the firewall) It should be able to work. Some shafts are designed with a slip joint which can be extended by hand and in some situations I have loosened the u-joint so that it can slide back a bit on the splines. -So long as it still has plenty of engagement. If you need to extend the shaft, there are a couple of ways this can be done. You can fabricate a spacer to go between the rag joint and the flange it attaches to, but better yet, you can get rid of the rag joint and have another u-joint welded in. When you have the u-joint welded in, take into account the extra length. If you don't want to fabricate your own steering shaft, you could buy one from Borgeson. If they do not carry one in stock, they can build you a custom unit with the correct length
-Any fan shroud mods needed?
Yes. You'll have to lower it. A kit should include the brackets. But if you make your own kit, you can make brackets with flat steel
-Will the automatic transmission shift linkage be fine without any mods?
It should be…however you might have a bit of bind when you go to shift. Make sure that when you install the body lift to check the linkages for any sort of misalignment, or interference then adjust accordingly. If you have a 4x4 with a manual shifter, it will need an extension welded in, unless you want the shifter boot pulled down thru the floor inside-out with the shifter knob half hidden.
-Which kit would you recommend? I found Daystar and Performance Accessories so far.
As mentioned, I prefer to make my own kits because I can do it cheaper… Heres how. If you have a local 4x4 shop nearby. They will sometimes sell new blocks individually or sell off used blocks from trucks they remove the blocks from. I've purchased new blocks for about $5 a piece. Some people really dislike body lifts, remove them and throw them away. If you're lucky, they might simply give you the blocks and maybe some of the other pieces which might work for you, like the shroud brackets. Hardware is relatively cheap. You can purchase the body bolts from a well stocked hardware store… just make sure you buy grade 8 for maximum strength. Shifter extensions are nothing more than short steel bars that usually need to be welded in. Any piece of steel will work (tool steel would be much better)
Bumper brackets. I usually leave the rear bumpers alone because it makes it easier to climb into the bed and I simply cover the gap between the bumper and bed with a piece of diamond plate. For the front bumper, a couple of short pieces of U shaped steel channel will work for the brackets.