2WD to 4WD Problem

I have a 1993 K1500 with 163,000 miles on it, 4 Speed w/overdrive and a NP 241 transfer case. I just started having an issue going from 2wd to 4wd Hi while still rolling. Usually I'm able to pull it right into 4WD HI at any speed with no problem. Now, I have to actually stop the truck, or be under 5 MPH before it will pull into 4wd HI without grinding. It's as if there is suddenly no synchro. I have change the fluid every 30,000 since the truck was new and the fluid level is right up to the fill hole. Any ideas?