GQ, by "roll bar" I assume you mean a chrome roll bar like you see in the bed of a pick-up truck? The Bronco in the movie "Oceans Eleven" had a double-tube roll bar in back, as did the Bronco in "Romancing the Stone". Looks awesome *******!
Or is it one of the factory roll bars that some Broncos came with? Smittybilt offered a roll bar, particulary for 78/79 Broncos, that fit under the cap. I don't know for sure if that will fit in your 84.
Keep in mind that the first roll bar mentioned is only for "looks" or mounting lights, and is not considered a true safety feature in a roll over. It's still better than nothing, though, so if it fits I would get that over the hitch. I plan to install a 10-point roll cage in my 84 Bronco. I have it in the "parts" room, just haven't installed it yet.