having to hold shifter for 4W

I will preface by saying that I am very new to 4x4 trucks and maintainance so i hope my question isn't to vague or too stupid.
I have a 1977 Full sized Blazer with a 205 transfer case, 1980 454 engine and a dana 44 with locking hubs. The truck runs great in 4 wheel drive, the problem is, i have to hold the shifetr in place. What do i need to do? Thanks
adam  

sorry it is a 203 transfer case  

I figured that was the issue. The best thing to do is replace the 203 with a 205 or 208. If you want to use what you have, the problem is in the shifter plates. I had the same problem with my old 203 and I just took the shifter assy off and cleaned it up and put it back on and it worked like new. It may not be the case on your Blaze but thats where you can start. I would start hunting for a 205 or 208 if it was myself. You can use the 203 gear section and find a 205 and make a doubler out of it. Thats the only thing the 203 is good for.  

thanks for the reply...does a 205 require any special mods to replace my 203?  

drivelines front and rear, possibly