i also experience the same thing, but that is because i have all season tires and never had a rear wheel car before

i find that once in awhile u get the feeling the back end is about to come out or feels very light at times eventhough i have two wheels in the trunk for added weight

my only work around for this is just take it easy and slow down

i've driven the same car with winter tires and same thing and i had gotten a 4 wheel alignment in the fall

hope this helps,
reny