I'm not a driver (yet) so excuse my ignorance but do both rear tractor tandems drive the tractor or just one??
just the rear, however most trucks have a switch on the dash that allows you to lock both axles for times when you encounter poor traction its called the power divider, when it comes to freighliners i know for a FACT that you should never use the power divider at speeds above 35mph, and NEVER engage it while the wheels are spinning
the rearmost axles are the primary drive axles unless you switch on the differential lock switch on the dash or the truck has ATC(like mine) which will engage the forward axles when the rears lose traction.hope this helps
