Parking brake binding
Last year I did a T56 swap and it started to give me problems like yours, it would be completely normal then next time I went to park it it would feel like it was binding up, only pull up about half way and not have enough power to hold the car on an incline.
It Bothers me when I have to leave it in gear because I kept my remote starter on it so I bypassed the neutral safety switch so I'm scared i night accidently hit the button in my pocket and start the car in gear.
Did you have another person to pull and release the e-brake while you looked at the calipers? If your caliper is sticking I think that could very likely be a problem I need to check that out on mine. I actually have an extra set of calipers that came with the rear end I bought I might just have to switch one out.



