What the heck is this channel looking steel stock piece that runs from the trans to the rear end on my 91 Firebird? Is it some sort of torque arm?

 

yup. thats the torque arm

 

ok, whats the purpose, especially on an anemic 3.1L auto equipped car?

 

keeps the rear end front rotating upward when you hit the gas, without it, there would be all kinds of drivetrain bind...it's a pretty essential part

 

is that because of the coil sprung rear end? I didn't look at it real close, but I couldn't see any UCAs on the rear. Does the torque bar act as a UCA in this application?

 

it's not because of the springs, without the torque arm, the car will sit with the pinion angle fairly negative, the torque arm does act alot like UCA's. We don't have the option to use UCA's becaue we don't have the room under the car, but the torque arm does serve much the same purpose as upper control arms.