Brine and injection??

The temp is probably mostly the culprit. For me, that 175-185 area is way too much for slicing and not enough for pulling.

I don't usually brine beef, but I doubt that had as much to do with it as the temp.

EDIT: The rump can be a "tough" cut to begin with sometimes based on the grade and how much connective tissue there is.

Even for well done I wouldn't go past 165. And thats AFTER carryover.

The rump roast is a tough cut of meat. My 'dear muther' would salt/pepper & flour it before browning in a bit of vegetable oil, then slow cook in the oven for an hour or so before adding a bit of water, putting the lid on it and letting cook all afternoon. She always made gravy with the pan drippings and generally sliced it very thin...she did not have good false teeth and liked everything tender.

So--- with that as my guide, I have browned a rump on the grill FIRST, then moved it to the smoker and let it slowly smoke for a couple hours at the lowest temp I could hold, I can't find my notes but I think it was about 180F. Then I wrapped it in foil, adding a bit of water mixed with lea & perins and some crushed garlic, and finished it in the oven until the temp was 145F degree. Let it rest and sliced it as thin as I could get it. Thin slicing is always gonna help you out of a jam with dry or tough meat.

here's a handy reference PDF from the Beef Council that lists various cuts and recommended cooking methods. http://www.beefitswhatsfordinner.com/CMDocs/BIWFD/Cooking Guidelines/Cooking Methods.pdf