Have an 81SD and last winter I had to tear apart the headlight assembly on the left side. The hi beam would not work and I traced it over time to the connection there.
There is a small plug in wiring harness between the lead wires to the h/l and the bulb itself. The plug in buss between them is where mine failed. Gets corrosion in there and fails. It is very hard to test as you almost have to take it apart to test it.