help! Problem shifting back into 2wd from 4hi

Can someone give me some info on why my 98 Blazer doesn't want to shift back into 2wd after being in 4wh (or 4 low for that matter)? I can hear it trying to shift in the transfer case but it just doesn't seem to have enough power to complete the shift. Thanks

I would first have the vacuum switch on top of the transfer case replaced.The part cost's about $32.00.The ball on the switch sometime's get's stuck in it.It worked for me.Otherwise you might need a new encoder motor.