I had this same problem on my 25 merc! I sent it to a very good offshore Race boat shop for them to do a waterpump, tune-up, etc... Well, when I got it back I had no reverse. THey played with the linkage some, but I still don't have the easy in, easy out like I used to. It would work if I really turned hard on the tiller, but I was scared I was going to break something. Never have gotten quite right. I have just delt with it! :-/
Many times it's the lower shift rod clamp that isn't
set properly, check to see that it hasn't shifted position
due to failure to torque properly. Many shops don't
because they have "developed a feel for their nuts."
(Hi Kemo, good line, had to use it)
Hey weedy, you should get that adjusted properly. It could wear out your gears or chip your clutch dog. its like brett said it is a pretty easy fix. Depending on the motor, you just need to put the lower unit in either forward or reverse and adjust the linkage so there is no tension/pressure on the linkage.
If you can find the connection location and type I can give you better direction.
Tom, did your problem just start by itself or has the lower unit been off.