where'd my 4hi go?

[remy]

was out driving in the snow today and discovered a new problem... every time i started going down a hill, and on some level areas, truck kept popping out of 4hi... i still have 4lo and it seems to work fine... t-case is a 208 behind a th400... any ideas?
it just kinda pops(thunk!) and then grinds until i shift the trans to neutral and then grab the 4wd shifter and drop it back to 2wd...

i don't understand it, and it worked fine a couple weeks ago... so, any help would be great... thanks


remy