Transmission problem?

I have a 99 Grand Cherokee, 4.0, 4x4 with a 42RE transmission. It sometimes knocks into reverse. Sometimes it's butter smooth, but more often than not it knocks a bit. Forward is no problem at all. I asked a mechanic about it and he said I needed a brand new transmission with no hesitation. I've done work on my vehicles, but i'm not a transmission guy, is he just trying to make extra money or is this really that serious a problem?