well it is hard to explain and obviously since i wasnt there i cant tell you. But on my explorer when i go from 65mph at 5 to 65 at 3rd (a much mroe extreme case then yours) the engine obviously will take time to match the road speed. Now if i let the clutch out too fast, the traction in the rears will break away for just an instant (because the engine was at 2000 rpm and now is trying to jump to 4000 rpm), but since i have my foot on the gas it is quickly followed by acceleration. This is bad for the car. Now i am not sure if that is what happened. I don't know what your throttle response is, and a jump from 5 to 4 isnt that much, maybe you just got unlucky or it happened so fast that you don't realize it happened Don't worry about it new u-joints should be sufficent
btw... are you sure, since you are running rich and all, that when u dumped all that gas into the engine that it didnt bog at all even for a split second, because that has happened to me, never broke a u-joint off it but it can happen when you dump alot of gas in at once, especially if you are going lean. (but it probably isnt this)