84 od tranny

just bought step side, ran fine until I got it home, tranny clunked when slowing down to a stop, also did this when taking off from a stop, put the truck into low & it seemed fine. Only seems to happen when motor warm, - it clunked & killed the truck- manually put in low & we took off fine. I don't have a manual yet, fluid is ok & doesn't smell burnt,, any help would be appreciated.