Remember that the Disco back axle will provide a HUGE extra braking effort compared to a Defender disc brake axle. It will probably apply as much braking force as the Def90 FRONT axle.
This is amazing for STOPPING (the general idea!) especially with a heavy trailer on, BUT, you will need to take care on wet and slippy roads as you will be able to lock up the back wheels if you brake hard.
Luckily for us permanent 4x4 people this won't happen as easily as it would in any 4x2 vehicle, but you do need to take care as even ONE back wheel locking up can be a problem.