Mine isnt cracked yet, but its a 2000... so im on barrowed time as far as it being replaced under warrenty. The weird thing is the gasket looks almost brand new on it, but its not an aluminum crossover manifold. I dont see why they would replace it with another cracking plastic one. Maybe the seal is just in really good shape or something...
Anyhow, how hard is it to swap manifolds on a 4.6? Looks straight forward to me... sits right on top of the engine, and with the air intake, throttle body, and plenum taken off it looks like theres alot of room to work with. This is just PI to PI manifold... not swapping from non-PI to PI.
And another thing... 90% of the 4.6 GT's ive looked at on car lots, and private sellers have had the aluminum crossover manfifold installed. Its a VERY common thing from what I have seen. Only ones I noticed that didnt have one are mine, an two other 4.6's with lowish miles.