I'm not sure when you feel the "clunk", but its possible its taking longer than you expect to engage. One note, on the NV transfer cases, maintaining proper fluid level is essential, as it has its own oil pump. Too little fluid, and operation can be sluggish or stuck in 2wd.
I'm also not sure why it feels like its hydroplaning. In 4wd, you should be able to feel the front end grip, at least some of the time. If it were actually hydroplaning (As Tbone suggests) you would be out of control and
4wd would be useless. If it is indeed hydroplaning, the likely cause is something other than the 4wd operation, such as tires, road conditions or speed.