Howl in 4th?

So this may be a little mix of 4 speed and / or rear end, but when I'm cruising along and drop the car into 4th it starts getting a little noisy. It only started after I put my new rear end in it, and while it sounds a lot like a rear end howl it only seems to do it in 4th. I can wind it a little higher in 3rd with no obvious noise, but the second I drop the car in 4th it starts howling. It only does it under load for what it's worth. Any ideas what it might be? I noticed with the rear end swap that the driveshaft now goes further into the tail secion than before, but I wouldn't think it would cause a problem since I can push it in further when the rear U joint is disconntected before it bottoms out.