This is exactly what happens to the tranny when you have a bad TCC regulator valve that is worn, which is almost always the problem with these valve bodies. This is the common sonnax part everyone talks about. The part is not that expensive, and is rather easy to install if you have the specific reamer that is required. The reamer though is expensive (Around $ 150.00)
You usually will get the common P1870 code, indicating trans component slipping, sending max line pressure to the transmission, giving you the hard 1-2, and basically slowly, yet surely destroying your transmission. Anyone who drives with the P1870 code and does nothing about it, should be prepared to rebuild their transmission, becuase it will eventually fail.
The TCC PWM solonoid can cause a harsh 1-2 shift also, but this can be checked with a scanner while test driving the truck. The scanner will verify if the PWM is within range. If its not within range, the PWM would have to be replaced. If its sending proper voltage values to the scanner, then the next thing to look at(assuming all other readings from the transmission are within spec. and no service engine light is on, is to look at the TCC valve regulator (the sonnax part).
The check balls are common, Trans-Go makes a thicker valve body plate that resists the common problem of check balls going through the valve body plate. The Trans-Go also resists the rusting, which is common on these plates also.
The clutches they are referring to usually are the 3-4 clutches. They are a very common problem, and burn up as a result of the TCC regulator wearing out in the valve body. A very advisable thing to do(especially if you plan on keeping it or if you do a lot of towing), is to upgrade the weak 3-4 clutches to something like the Raybestos Z-Pack, or Alto 9-clutch pack. They can take the higher stress in these trannies, and last a lot longer than OEM. That being said, the O.E. 3-4 clutch pack has been redesigned to take more stress than the previous ones used. In my opinion, I still don't think they are good enough.
Another good thing to do is to just replace the sun gear shell with a stronger, better built unit. Something like "the beast", or something stronger. This is a very weak point in the 4L60e.
Ask and see what they replace, and what they leave in the tranny. Some builders will replace bushings, bearings, all clutches and stuff, others will just replace whats broken. This last option is not a full rebuild, just a fix(hopefully). Either way, I hope this helps give you an idea as to whats going on.
Also, here is a link for the Sonnax TCC regulator.
http://www.transmissionspecialty.com...-Combo(rb).pdf