2006 Kenworth t800 need advise

Quick version.

normally only one axle drives. think 2 wheel drive or one wheel peel!
lock the power divider and both axles will drive, similar to putting a pickup in 4wd. two wheel peel!
Full Lock and all rear tires will turn at the same speed! 4 wheel Peel!!!! only to be used when really, really needed!! preferably in a straight line,
The truck will want to go straight, may not turn,
Also if theres enough traction, and you turn too much with enough power you can break an axle shaft!!

Personally I like Full Locking Rears, but for the average truck they aren't really necessary.,

 

Not trying to nitpick or anything but the only one axle drives thing is a common misconception. The power divider is basically a diff between both axles and behaves the same way. On a stretch of good road, each diff will receive 50% of the power and divide that to each wheel, basically giving each wheel 25% of the available power. Any one wheel slips on either axle and the majority of power will direct to the spinning wheel. The power divider lock eliminates the differential action in the power divider, which causes both axles to have a 50/50 power split at all times, rather than only when there is perfect traction.