Diff/driveshaft "slap" ?

See original thread here:
Having some issues here fellas, any help is appreciated. I don't want to start just throwing parts at it if I don't have to, ya know?

For the clicky impaired:

Hey guys, just a quick tech question here.
Little info first, it is a 2009 tacoma regular cab 4x4, lockright rear locker, 5 speed

After leaving work tonight, I heard a "slap," alomost like a flatspotted tire,coming from somewhere beneath/behind me. It goes away when either the clutch is in, trans in neutral, or coasting in gear. It is speed dependent, and the gear I am in doesn't matter. The slap seems to get faster, the faster I go, but goes away when I let off and coast, even in gear.

When taking off it doesn't quite seem like it has that "engagement/grab" feel like when the locker engages. Is it possible that the springs or something are worn out or broken? I took it up a ramp on the way home and I could lift a wheel off without slipping a tire in 2wd. Could it be a u joint going bad? I thought I detected a minute amount of play in the front u joint of the driveshaft.

I'm kind of stumped and I am going to park it until I figure it out and can get it fixed.
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I would look at a U-joint first, that would be your cheapest route. I had the same sympton your describing on my jeep and it ended up being the u-joint. The needle bearings were worn down/non-existent on the one side of the cup.

 

Well, this morning I went out for some investigative work. I checked the slop in the driveshaft, and it had a good inch and a half of rotational slop before you couldn't turn it anymore. Is that normal? I think the front u joint is bad, but couldn't drop the driveshaft to drive around in 4hi to check. Talk about overtightened bolts.

 

Well, I dropped the rear driveshaft to locate the proble fore or aft today. Drove around, and sure enough I couldn't get the noise to rear its' ugly head. So, I went ahead and replace the u joints and put the driveshaft back in. Took it for a drive, and guess what? Still had that damn noise.