1986 Blazer won't shift out of 1st

I have a 986 Chevy Blazer with a 305 and I think the 700-4 transission? I know it is a 4 speed or overdrive trans, at any rate.
I recently got this running again, after it had sat for awhile, and now it is not shifting out of 1st gear.
I can get up to 25mph or a little over, and it just stays in 1st gear.

Is there some sort of vacuum line or a switch on these that could cause this, or I am just looking
at a having to do a tranny rebuild?

The reason it sat for awhile is because I left it at a storage site, and the carb got stolen, along with a hole punched in the gas tank.
So I had to get some things fixed before I could run it again, but now I would like to get it running, if I can.

I checked the trans fluid, and it looks a little brown, and it was a little low, but I have added trans fluid, and it still doesn't shift out of 1st gear.



Thanks for the help!!!

Brown fluid = not good. 

Maybe try dropping the pan and checking the fluid and filter that comes out.  If you see any metal/silver spec, you're fubared.

You might want to check your vacumm lines to. They may have broke. Check the shifter link on the trans it may have come loose or some thing.