I'm in similar situation: just put sure grip 8 3/4 together with 3.91 gear and been driving the last 2 days in my 71 demon... noticed the chunk gets pretty got after I've been driving (hot to the touch) even tho its only 70 degrees outside. I'm running 80-90 wt gear oil... What oil are you using?
PS - I'm not using any kind of additive but it sounds like I guess I should...
I have a 8 3/4 rear with a sure grip and 3:55 gears, i used the lucas full synthetic 80W-90 gear oil and the mopar additive.
Honestly ive never felt the 3rd member after driving to see how warm it gets.