dana 30 spiders won't work. i wouldn't advise you to replace your old spiders with a locker either. locking a rear D35 is asking for problems.
you've got a couple options. search the boneyards for a D35 and take it's spiders. go for the posi-locks since that's what you have.
if you've got some change, upgrade to a D44 rear. Explorer 8.8 axles are cheap, but require some modifications.
you could purchase a Dana 35 Super Kit and seriously beef up that rear.
but my advice? if i were you, call around to all the 4x4 shops ans axle shops near you. i'm positive someone has your spidergears just sitting around collecting dust from people upgrading. they'll be cheap...possibly even free.