In a nut shell if you hear a shift about 20-40 feet out at WOT then that is 2-3 shift. If you here a shift from inches to 5 feet out or so that is the 1-2 shift. If you are unsure still then read on.

You can lock 2nd gear by adjusting IN the set screws all the way. Look in the manual and it will show you where they are at. Make sure to count how far they need to go in. The standard setting is 5 turns. Also make sure that you are adjusting the correct screws. These will be on the sides not the easier to get at straight on ones. If you are unsure, than it is best to leave it all alone. Basically look at the manual for which screws to adjust. By tigtening these 2 screws, it won't allow those shoes to expand, thus it will be locked out. Of course when you are done, put the screws back to where they were.

Also, it is a great time to use 500k diff fluid if you can get it, if not then locking the center is also fine. This will give you power to the front and back at the same time. In the box there should be a small metal thingy. About 1.5 inches long and probably 3/4 in wide. Just remove the spider gears and place that in instead. I think that this is also in the manual. Now before you think about opening it you should know that those screws for the casing have permanent thread lock on them, and it will be a royal PITA to remove them. Somebody on here recommended the oven set to 250 degrees for 7 minutes did the trick.

When in doubt, come here and ask questions.