Strange shaking problem while in drive
 

I have a 2004 Focus with 70k miles on it, recently it developed a strange problem, when i put the car in drive it starts to slowly shake worse and worse over a 40-60 second time frame, it gets really bad but if i put it back into neutral or park it immediately goes away. It does the shaking in reverse as well. It seems to run fine while driving, no smoking or anything, and the transmission has enough oil. I don't know what to check any suggestions?

What kind of maintenance has been done to it so far ?

that sounds like a bad motor/tranny mount? Since as you stated when you put it in nuetral it stops.

A easy way the check this is with a assistant watching the motor{in a safe place} While you load the motor. If you see movement out of the ordinary thats probably it.