89 B2 t-case slipping out of high range

[scubacy]

Getting ready for our first snow, and I locked in the manual hubs and put it in 4 high (manual also), just to make sure everything works... and around 45mph it suddenly jerks a few times and then drops-out into neutral. I stop, put it back into 4 high and same thing after only a minute or two. No "4wd" light on the dash either, the 4 low light does come on though. So, I don't know if it's just not engaging all the way or what going on.
Anybody have any ideas?

Thanks