whats the purpose of all the suspension stuff in back. I know the adj. torque arm changes the angle to provide more rubber to the road. but what about the panhard rod, and trailing arms, what do they do. and is square or tubular better (best brand).

on the torque arm is there anything to pay particular attention to or do they all do the same job

 

All that 'stuff' is pretty important.

The panhard bar stops your rearend from moving left to right when moving. The pbar usually only needs to be paid attention to if you lower the car. The lower ride height will change the geometry of the car, and move the rearend an inch or two to one side.

The control arms, they transfer majority of the energy from the rearend to the body to make the car move. If you're into drag racing, then stronger boxed or tubular control arms will give better traction as they tend not to twist like the stock control arms. And if the car is lowered, control arm relocation brackets can change the angle fo the arms to increase straight line traction.