FWIW, it looks like the MD-88s start appearing in SLC on May 14:
MSP-SLC DL1457 Lv MSP 935p Ar SLC 1127p
SLC-MSP DL1456 Lv SLC 835a Ar MSP 1208p
Based on that, it looks like the plane will overnight in SLC. The -88s on MSP-SLC look to be short-lived though - they disappear first week in June. When that happens, you do see 757s appear (the 757 replaces the 737-800 on the morning MSP-SLC; an A320 replaces the MD-88 at night on MSP-SLC).
Addition - checking in mid-May, this will be on the only MD-88 flight that SLC will see. All other mainline flights are 737-800/757/767-300/MD-90.