Transmission shifting extremely hard

I have a 2006 Chevy 2500HD with about 180000 miles. All of a sudden the transmission is shifting extremely hard at times. It doesn’t shift this way all the time. What could be causing this problem? It doesn’t have to be under hard acceleration either when it does shift hard. It sometimes is during hard accelerating and sometimes it is under very little acceleration, but as I said it doesn’t do it every time. When it does shift hard, it seems to do it in all gears.

No Check Engine light?

Usually when a transmission shifts hard it means the computer has put it into the limp mode. The computer raises the line pressures in the transmission to prevent it from slipping and this causes the hard shifting.

If there is no Check Engine light, then you might want to have a scanner plugged into the vehicle to see if there are any pending codes.