I've got a completely stock 75 K5, all well when parked. After about a year I went to move it and now it will only go in reverse. When shifted into drive nothing happens. Reverse works great! I've heard of an O-Ring in these transmissions that could be the problem. Have yall heard of this?

I had the same problem on my 93' s-10 blazer this past summer,
this is why (http://johnpsu.web1000.com/pics/cars/tranny4.jpg)

Hope that helps!!

Thanks John!