Yes, it was before the mid 80s. The SFZ operated over UP's Overland Route, which goes through neither Salt Lake City, nor the Colorado Rockies. Salt Lake was only an intermediate stop for the Desert Wind. After 5/6 shifted to D&RGW, it split at Salt Lake. Not sure how they handled it during the interim period it was officially supposed to have been routed on the D&RGW, but it was "detoured" on its former UP route due to the Thistle slide in April 1983. During the "detour" period I don't recall whether it continued west on the SP over the Lucin Cutoff as it did before, or whether it went south from Ogden to take the former WP as it did after it started running over the D&RGW. My guess, and it is only a guess, is that it stayed on the SP because there were no intermediate stops between Ogden/Salt Lake and Elko, in paired track territory, so there wasn't a passenger impact on whether it went over the ex-WP, recently merged with UP, over Silver Zone or SP over the Lucin Cutoff. Operationally it is just easier to stay on the UP/SP Overland Route, so the split would have remained at Ogden. However, if they shifted to the ex-WP and ran the whole train south to Salt Lake during the "detour" period, the split could have been at either Ogden or Salt Lake. I don't think they did, but I don't know. It doesn't make a bunch of sense.
I also don't recall exactly when the new trackage at Thistle opened and 5/6 actually started using the D&RGW, but it was fairly quick for a construction project of that magnitude, so late 1983 or early 1984 is my guess. I have a couple of books that have the date it started over the D&RGW (which was a big deal to Amtrak), but I didn't dig them out. But it was in 1984 at the latest.