The engine braking is normal and I would expect it to only occur when the trans is warm. On level ground, not sure about that. Mine has to be on a certain degree of grade to do that.

 

Its normal and it has more to do with vehicle speed than it does anything else. Its not going to engine break/downshift when you're running 70 miles an hour and let off the gas for a few seconds. Mine seems to do it mostly when I'm coming off the highway on an off ramp and costing/soft breaking to a stop.

 

Usually you need to tap the brakes for a second going down hill before it kicks in.

I have sworn that it causes a decreased gas mileage due to the higher RPM range when its kicked in.

 

Perfectly normal, its part of the setup on these trucks...actually I believe some other manufacturers have something similar now as well. It does get a bit annoying and it does suck up more fuel. However, all automatic transmissions will upshift and downshift on their own during certain acceleration/deceleration occurrences.

As for holding gears longer, yes it does that too sometimes, especially when its still cold. Tow/haul mode will cause it to hold gears longer and downshift sooner as well. The transmission won't shift into 5th or 6th gear until its warmed up. So if you are driving along and its holding in 4th its perfectly normal when its cold. This is the only truck I know of that has both a transmission oil warmer and a cooler LOL.