Engine shakes BAD!

[Dus]

SE-R gurus,

Today I started my girlfriend's 1998 Sentra Sr20de (automatic) to go to
work, the engine starts to shake after 30 seconds.  If i give it gas and
hold it above 2K RPM, the shake goes away.  the shake only shows anywhere
from idle to 1.5k rpm.

the car has 67K miles on it.  the engine shake just comes out of no where,
no symptoms of it the day before.  I didn't drive it thinking the car might
get worse and i can't get to work in time.

any idea what could it be?  it's a daily driven car, never been in any race.
the way the engine shakes is from front to back (from front of car to
firewall).  the engine idles smooth, sounds good, no stumble or rough idle.
however, the engine shakes the whole car the "bobble head" doll in the car
can't stop shaking its head.

thanks in advance.  i'll be doing a tune-up on it as soon as i find time
over the weekend.  in the mean time it won't be driven anywhere till i find
out what the problem is.

dus