'98 Silverado - Difficult to engage 4x4!

I've got a '98 silverado, 2500, 4L80E, 4x4 with the manual shift four wheel drive lever. I cannot get into 4H if I'm over 5 or 10 mph. That's not with a load on, driving uphill, stuck in a mud pit, or anything else (just coasting on flat ground or going downhill) It's not a big problem, but there are times when I get into snow on the highway and would like to engage at 20 - 30 mph (or any reasonable speed greater than 5 mph). Any ideas?

Thanks!