There were a handful of commercial AM rockers in the 90s. A few off the top of my head:
970 KBBT Portland, OR 1990/91 to 1996 (alternative - "The Beat")
1060 KUKQ-AM Phoenix, AZ 1989-1993 and 1994-1996 (alternative)
1330 WEBO Binghamton, NY - years uncertain, but possibly into the early 00's (alternative - "The Web")
1340 WLAV-AM Grand Rapids, MI 93-95 (alternative - "1340 Underground")
Of course, some college stations have been on AM - for example KUOM "Radio K" Minneapolis which plays a great deal of alternative rock and has since 1993, even showing modestly in the ratings at times.