Search for threads on the governor, it is well known for causing a 1-2 hung shift where you have to lift to shift. Had it happen to mine so I followed the thread in the transmission section to replace it. If you can manually shift from 1-2 normally that is likely the problem. The governor is sensitive to debris and dirty fluid, some people think you can get away with removing the valve and polishing it up. That may work for a while but for the money (cheap) replacement is a better idea. You have to pull the transfer case but it is easy to get to. Couple hour job. There is a heavier weight available too, makes for a slightly lower rpm shift when not under load, I like it.
The 2-3 shift is usually the modulator. It can be replaced through the hole in the floor from the inside and is cheap to do. There is a thread in the transmission section. Usually your transmission fluid will start mysteriously disappearing on you when the modulator fails because the engine sucks the fluid out of the transmission through the vacuum line going to the modulator and ingests it. That burning of transmission fluid can sometimes contribute to a spark knock too. So I have been told anyway. Took me about a hour and a half I think. It was a while ago.
If you haven't pulled and cleaned the valve body that could be the cause as well, the sediment (black crap from the clutches) can gum it up. If I were going to remove it to clean it I would rebuild it using Glacier's rebuild thread (sticky) in the transmission section. Rebuild it or replace it with a rebuilt unit. There are many improvements you can make to the VB, the parts are included in one of the two shift kits Glacier uses, as well some are available from Sonnex. Read the rebuild thread, it is really done well.
If you want your trans to last for a long time you have to keep it from getting hot. Drive it around in "D" unless you are above 50 mph, never tow in "OD", and if you are going to tow you should install a big transmission cooler, I have two. I also installed an external filter to catch even more of the smaller particles missed by the factory filter. When you replace the filter (in the pan) get a Fram Dacron filter. Normally filters are not much more than a screen, so the Dacron catches a great deal more obviously.
Oh, and welcome to the site!