try shifting to 4lo from 2hi. me personally i have never shifted from 4hi directly to 4lo i've always gone to 2hi then 4lo. see if that works
there is 2 ways to get to 4lo. put it in neutral and then push 4lo and wait. or put it in drive and DO NOT GO OVER 3MPH and then push 4lo. 2nd one is the recomended. the 1st one takes longer

 

Well back to square one. Left this morning and put the blazer in 4wd. Everything fine. Started going down the road and the service 4wd light comes on. I tried to put it back into 2wd and nothing. I stopped, shut the truck off and restarted. It went back into 2wd fine. Started going down the road and all idicator lights went out. I'm assuming I blew another fuse but haven't checked yet.