Manually shifting turbo 400

[crawdad]

I started to jump in on the other thread below, but thought it best to start another since my problem is a little different. My transmissing will pull and shift fine as long as I have it in drive. When I was driving it home right after I bought it, I noticed that if I pulled in down into a lower gear, it didn't seem to make a difference. If it was shifting down, I couldn't tell it. I did notice after I got it home that it was a couple quarts low on transmissing fluid. Would that be the likely cause of my problem, or do I need to look at something else. I've since put fluid in it, but haven't taken it back out on the road because I don't have a tag for it yet. Any ideas appreciated.