SOUL ASYLUM: Grave Dancer's Union (Columbia Records). WHILE Nirvana finds itself thrust upon the ''Alternative'' music throne and would-be accessors ride on the band's flannel coat-tails, Minneapolis' Soul Asylum has been busy perfecting a potent combination of punk riffs and pop sensibilities that predates Nirvana by a good five years. Its latest release, Grave Dancer's Union, should finally garner the overdue attention Soul Asylum deserves. And not because it cashes in on the present fascination with all things ''grunge''. There is plenty of crunchy guitar, some aggressive song pacing and a few anguished screams thrown in for good measure, like on 99% and the stomping Someone to Shove. But Grave Dancer's Union delves deeper, revealing a band that has matured, varied its sound and in the process discovered its true talents. Fans of Soul Asylum's more raucous earlier efforts will no doubt be disappointed by the fact that the band occasionally adds strings and piano and (gasp) turns off the amps. Songs like Runaway Train and New World, coupled with more noticeable production,soften some of the band's raw edges. But the new direction fits with the tone of frontman Dave Pirner's sharper, darker lyrics. Grave Dancer's Union is a solid, multi-faceted album that draws on more than volume for its impact.