It was much worse on mine at -7* as well. The resonance was at 37 mph and at a frequency higher than wheel spin (so definately not the tires). So far having them change all the engine and trans mounts seems to have curred it.

When you take it in, have them check the trans mounts first. I am nearly positive this is a loose mount that is causing the drive shaft not to be well aligned -- then with everything stiff in the cold weather, the shakes come on bad.

Good luck.