Can some one give me some idea as to where to start a search, on my transmission issue?

The culprit is my 84 300TD. last filter and fluid change was 20,000 ago. It has been running well until today. on the way home it started slipping when it would down shif into 2nd. I pulled in to a safe place, and checked the fluid, which was abit low, added about a quart. (first time I have ever added fluid, although I have not checked the level in about 7,000 miles)
the car still slips in first, and in second it slips and shudders a bit. If I can coax it to shift in to third then it runs fine.

BY accelerating and letting off repeatedly.. once the car is rolling it gets slightly better and the transmission will kinda grab hold at 5 mph, then it shifts into second with a thump, but chatters and slips until third.

in fact I drove it home about 35 miles in 3/4 gears with not a problem, but 1//2 slip like mad.

My question is it this a adjustment(some how I doubt it) or a new transmission? The fluid did not seem to be brown, or burned smelling. As I write this may be the only way to find out more is to put a new transmission filter and oil in it? please give me a topic to search under and I will run with it, but I dont really know where to start.
Thanks
Jason

__________________
46 WDX Power Wagon

84 300TDT daily driver