I had a similar problem with my 97 Sierra.
Pull the battery connection for about 20 minutes. This will reset the computer and the transmission computer. If the vehicle shifts normally after this, then the problem lies in the tranny.
Most likely the valve body is dirty and/or the torque converter is starting to slip.
When a problem is detected by the computer, the transmission told itself to shift hard to (believe it or not) protect itself.
If you bring the truck into a dealer for a service, have them read the error codes and this will help track down the problem.
Good Luck
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Tuesday, June 12th, 2007 AT 2:59 PM