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The TH400 is the most difficult transmission out of all the major manufactures. Actually the whole GM line-up is more difficult then anyone else's. The 4L60E, 4L80E, 6L80/90E all are difficult transmissions. The upside is they are also the strongest transmissions. That is why racers use them almost exclusively. I recently just rebuilt my TH400 to handle up to 800HP continuously and it only cost about $600.00 . You can make this transmission handle crazy power if you have enough money.
If it is all greasy and nasty with a red tag, think "who would have pulled this just for the fun of it". It needs rebuilt.
I have a 86 civvy 3/4 ton that I drive a heck of a lot. It has the same transmission (th400). It (the whole drivetrain) is geared rediculously low like the cucv. Like was previously said, you can't feel it shift from 1 to 2 unless you shut up your shotgun, and radio, and really listen (feel) closely. It shifts into 2nd at 15 mph and 3rd at 30. Feather the gas and it will be sooner. All she has is 3 so you will only feel 2 shifts. My top speed is 55 but I only go 45 because it is a 350 gasser and it is wound too tight for my pleasure if I go 55. Not a highway truck. I avoid it like the plague. In my days I have never seen a 400 tranny go out. I did however one time have one that was stuck in 1st for a long time unless you honed it and then it would upshift. Took it to a trans shop and the guy screwed me over and didn't fix it as it only worked for a few days. So one day I deciced to drop the valve body to make sure all the ball bearing check valves were in place to find out the entire valve body was loose. Every bolt! I dropped it anyway to assure the bearings were in place and they were. Put it back together and drove it for nearly ten more years before my son totaled the truck. I have also had plenty of vacuum lines rot, break, come unhooked and always look at this first. Burnt fluid usually leads to slippage (actually slippage leads to burnt fluid) and not once have I seen slippage in a 400. However, not once have I seen a 350 trans not fail with hard work. Only word to me for a th400 is 'bulletproof'. If gov knew to use it for us, this only reinforces my viewpoint as well as all the dragstrip folks I have known over the years.
Def take heed to that.
Dropped the pan and there were only very fine shavings like sand which was very comforting. I dropped out the valve body with the tranny still in the truck only thing you need to watch out for in doing this is the six 5/16 ball bearings dropping out. 
