Quick clunk sound coming from the front half of my truck.

Ok. So 2000 f150, 5.4L V8 4x4. I have a clunking sound coming from the front half. I can give a quick and moderate thrust on the gas, then clunk but more of a pop sound. That's when im in reverse. Also going over pot holes, depending on the size, its not to bad. But there's a stop sign I stop at everday, and there's chewed up pavement as you take a right. And good lord, when Im slowly gassing it through, it sounds like somethings about to snap. That brings me to my theory that it could be my transfer case or front diff. Now I spun the front driveshaft on a '98 & I think an '02. Both spun freely, like 5 complete turns worth. Mine does like 1/8 or something like that, then stops. That's in both directions. I didnt install any hub-lockers on my truck and neither did my 2 friends on their trucks. What's the deal? Is it the trans case, front/rear diff, transmission. I think I know its not anything suspension because when happens, I feel a very slight tap on the manual t-case shift lever. I looked it up and have a borg-warner 44-06 (t-case), took it off I think a navigator from a scrap yard, and stripped all the parts out that tcase. Only problem is, its an electric shift model. Mine's a manual. Just need to know what you think about my whole predicament. Any questions/answers email: thomas. Savard@aol. Com