"You hit it on the head. Shocks aren't controlling the rear housing. Couple this with the fact that with stock rear shackle location the spring may be stretched full out. The rear shock may be to short topping out. The rear shackle likely needs to be moved forward or replaced with a slider(also forward). get the correct length shock. Talk to the supplier about valving options. I'd bet a stick car needs stiff.
Doug"

Thanks you guys. I'll disconnect the shocks, let the rear end hang down fully and verify my shocks are not restricting the extension. I'll try and get a picture of my rear shackle position, and look into installing sliders to replace the stock shackles.