Car acts like its in park when shifting to reverse

[keenan]

I've got a 2002 Tahoe with 118k miles. It runs great but today was the second time this happened to me.. When I'm in park and go to reverse it doesn't move when I give it gas, its like its still in park. Then if I go back to park then reverse again it will work perfectly fine. This has only happened twice, today and yesterday. Anyone know what's up?
Thanks!