Transmission wont engage?

Yes, I have a question and am wondering if you may know what the problem is. I have a 1978 Chevy G20 with a 350 engine, 5.7 Liter, 3/4 Ton, 8 Cylinder Van with a Turbo Hydramatic 350 Transmission. Okay, my transmission will not go forward, but works just fine in reverse. The last time I drove the vehicle was in October / November 2014 and when I drove it then it shifted perfectly without any problems. Meaning it has sat not being drove for roughly 4 months as it is now March 2015. I checked the Transmission fluid from the dipstick it was full, translucent red, and contained no metal fragments. I always make sure to check my fluid in it every time before I go out in it and it has always been at the full line. Also, when I say that it won't go forward. I mean that it doesn't engage in any of the drive ranges such as (D), (L1), or (L2), but Park, Neutral, and Reverse all work fine. The only leak I have seen is a very small leak is around the speedometer cable. Where it leaks from the speedometer cable is also translucent red transmission fluid.

I have probably drove the van about 200 or so miles since I have owned it, and I have owned the van for roughly 4 or 5 years now. The van itself only has 87,000 miles on it and the previous owner took really good care of it and documented every oil change, etc. It has never given me any transmission problems in the past. I have already changed out the Transmission Modulator and that wasn't a problem. Now I am wondering if the filter maybe clogged and not letting enough air pressure or fluid through to engage the forward drive ranges (D), (L1), and (L2), or if The Valve Body could be clogged some how.

I have scoured the internet and people have said things like The Forward Clutch / Clutch Packs are burned out, Forward Sprag messed up, Snap Ring broke, Sun Shell Gear, etc. Though some of the questions people have asked have different parameters than my situation does. I haven't heard any loud bangs, pops, or anything like that from the transmission, and I haven't smelt anything burnt from the transmission when I have drove it. I mean I honestly don't see how any of the things I have listed in this paragraph can go bad unless they can do so while not being drove. I don't race it or anything like that and haven't even taken it out on the highway. Just very local driving even when I have drove it.

I have heard seals can also go bad as well internally. I got the transmission hot and let the van stay on for 30 minutes yesterday and since it was good and warm here. I put some Lucas Transmission Fix in it and gave it plenty of time to circulate through the transmission and I put it in hot because that stuff is like molasses, and it didn't move forward still. My dad came over and checked the linkage through visual inspection and said it appeared to be doing right also.

However, at the very best I am a shade tree mechanic. I have tried to be as thorough as I possibly can in describing everything. I hope that I wasn't a bore with these long paragraphs, but I figure to properly assess a problem that any information even the most trivial might be able to help.

Through the research I have done I am leaning in the direction of changing the filter / looking at the valve body. I am wondering if the Filter can get so clogged that it wouldn't allow the fluid and or air pressure to reach the forward clutches and not allow them to go forward? I was looking at the Big Box Automotive Parts Retailers and the Transmission Filter that fits this product according to their description is supposed to be a nylon screen rather than an actual traditional transmission filter. Would that make it more prone to be clogged?

I appreciate anyone who takes the time out to read over, review, and answer my inquiry. Any help or suggestions would be greatly appreciated. Thank You!