some one at work was telling me to check to make sure the rear end doesn't get to warm. so after getting home from the girlfriends house 30-40 minuets away i put my hand on it. i could keep my hand on it for a few seconds felt pretty warm, not to bad. how cool should the rear end be

If your not getting noise or vibration from the rear end its good. Just make sure it has lube.

there is a humming at higher speeds, 75-90, could it be the drive shaft

I guess it could be the drive shaft but, humming is more like a rear end noise. especially if the gears were just replaced. Tires are the same as before, not the cause?

is there anything to do to fix it. if it is the drive shaft how would i diagnose it.

If you can keep your hand on it for a few seconds you should be OK. The reason you don't want it too hot is because heat will toast the gear oil, which causes it to lose it's lubricating properties... It takes a decent amount of heat to toast gear oil.

I know that when you start doing some serious wailing you need to add some extra cooling... hell Ford sells a diff cooler for the GT :D

Your temp should be ok if you can keep your hand on it for a few seconds.

Use synthetic fluid to combat high-temp viscocity breakdown, but I assume you already are. And I agree with above, whining is most likely gears. The Yukon gears in my ride whine more than stock, but I knew that going in so I kinda expected it. No biggie.

Does it whine under accel or on decel?

It dosent whine. It hums around 70 85

But does the hum cange under accel or decel?

When you let off the gas it goes away, but when you accelerate over 70 it gets louder

Would it be safe to drive to Detroit or am I just being paranoid