yeah...the differentials in our trucks are the clutch type. They only spin posi when they have a load on them (ie pulling a trailer), when traction is important.
The thing is, when your doing a burnout, yes theres an initial load on it, but, its not fully locking.
Now, try getting stuck on a thin film of mud (I did it...lol)...only 1 wheel spun, and I have a 3.55 Posi.
So, you could upgrade, or, just slap a dana in there, and get a bullet proof rear while your at it