It's not that it has done this since you purchased it, but more that it's doing it now. I'll assume you have a stock differential. A lot of problems occur when it seems the engine doesn't have as much power as it used to, which would cause you to shift too early. When the cat on my 89 rotted and was pretty much clogged it caused the same problem. Replaced the cat and the power increased to normal. I also replaced the muffler a few days later just to have the best flow, but the problem was the cat. Prior to replacing the cat it ran fine just lacked full power. I think you should disconnect and rule it out.
I`m not quite sure what you mean either, but normally when a cat goes flat (goes out) it results in a moderate to severe lack of power, no matter what gear you`re in.
From what I make of it, it almost sounds like the transfer case is stuck in the low range.
Easy fix, take the short shift lever near your right leg & shove it forward. After it stops, it will be in "4wd High Range". Now pull it to the left & shove it all the way forward. You will now be in "2wd High Range".
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