First check would be the upper and lower engine supports. The lower one in particular is weak and breaks.
Here's a way to make it more secure and save some $$ in the process. I did this (and the upper mount) on both the cars in my sig and it fixed this and other transmission shift issues. It will also usually cause what is called the 'garage shift' - where when moving from N to D or R to D (like leaving a garage from sitting), you will hear a "clunk."
Post #30 here gives a nice DIY fix for the lower mount/support:
Decide to check this torque mount after reading about it here. Both my V70 T5 (110k miles) and XC70's (69k miles) mount looks like this http://i49.photobucket.com/albums/f258/howardc64/DSCF1571.jpg Looking at how much rubber there is that actually takes the load, it doesn't look like it...
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