When I turn on the a/c there is a low knocking noise in the rear l/h side of the truck

This fault is a bit srange, when you apply the brake pedal you here a
"Knocking" noise from the front of the car, alternating from one side to the other, the cause of this is a bad earth in the rear light clusters,
which results in the headlight motors activating "Jumping" thus causing the noise, solution is to clean the conections and conecting
blocks of the rear light clusters in extreme cases change the rear light clusters but a good cleaning should fix it hope this was helpfull

jlarson737, did you ever get a solution for you knocking noise? I also have a 2008 Aspen and have a clicking (knocking) noise coming from the 3rd seat passenger side of the Aspen.
Rod

Get someone else to bounce the car while you lay on your back and look. We have no idea what the problem can be and you could have left a lot of stuff loose under there. Maybe you didn't tighten everything up. Check it with a friend bouncing the car.

the brakes were not installed correctly or something broke hardware wise

rear suspension mount rubbers , loose rim on hub, wheel bearings are all possible problems

Well furst of all fix the leak before you have a fire.

I agree, sounds like the adjustment screw got off balance some how. I doubt its the wheel cylinder. Could be the bearings Compare to other side.

jolt from the rear of the car when the auto gear box changes in low gear is a common fault in the older s types. most of the time is a sign of the gear box on its way out. is worth checking gear box oil level and oil quality though. noise from near side rear would probably be the damper rose joint which make a knocking noise when they wear.

Your axle bearing is more than likely shot. The right side goes out first for some reason. Fairly inexpensive if you can do it yourself.

bad ball joints