1999 gmc sierra 2500 4x4 6.0l with 247,500 miles. Transmission tends to have this problem when trans temp reaches 170+. Cruising at 75-80 on cruise control will cause trans to downshift normally upgrade, usually to 3rd, but will not upshift on downgrade. Stopping truck, turning off ignition, restarting after a few minutes, and starting up slowly will cause trans to shift properly until next upgrade. If cruise is not used and rpm and speed are allowed to bleed off going upgrade, the trans will remain locked up. Am willing to replace trans, but want to make certain it is trans. Thanks