I think I read some where that this was a normal condition. It will allow the temp to rise in the transmission fluid before allowing it to shift into 6th. Mine does it also.

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07.5 3500 Quad, SRW, 4X4, Laramie, Leathered, Dark Metallic Gray, 6.7, 68RFE, 20K Reese, Ride Rite's, 2 Viair air compressors, Bully steps, Kleinn Locomotive air horns (there was 2 and now there are 5), Billet polished locking fuel door, 'A' pillar mount Auto Meter Pryo, PML X-deep trans pan (Yet to be installed)
06 Durango 285 RL 5'er Lifted by KZ
96 2500, Ext, 4X4, V-10 Gasser @ 400 HP, Gibson 3" single Exhaust, Ride Rite's, 285 Bridgestones, 170,000 miles, Faded Green, Carr steps, Brush Guard, 2 - 385K CP KC Daylighters, Dual Batteries, On board Air system