Keeping in mind I am only stating what was on the books as specified for a particular year, and drive line combo and that mother Ford has a reputation of instaling what ever is at hand and who knows what has been swaped on a vehicle you bought used.
1978 Broncos came with one of three different rear drive shafts depending on drive train 1310 series on both ends unless equiped with the 400 eng, then it was 1310/1330
1979 Broncos came with one of three different rear drive shafts depending on drive train but all used 1330 series joints at both ends EXCEPT when equiped with a 351M/auto trans/NP205, then it was 1310/1330 5-153x (1310) at the pinion and 2ea 5-278x (1330)at the TC.
I fully admit this is all book data, and what is under your bronco may differ, but for what it is worth there it is.