I apologize for the delayed response in this matter as I am a full-time student and work full time as well.
I have recently scrapped a 98 Plymouth Grand Voyager SE. I had the problem of tranny leak, and it began to slip. After nearly a year of the van sitting, I finally had money and time to figure out the problem. First thinking it was the seal and/or torque converter, of which I purchased, I later found it was the transmission itself due to the leak (seal). I dropped the transmission pan to drain the fluid and found hand fulls of metal shavings.
I suggest having this seal replaced, and the transmission replaced. A brand new one, as I have priced them is around 3-5,000 dollars. I reman was about 1400 and a used 800. The only thing about going with reman or used is that you could very well end up with the same problem.
So here is what i suggest. Have the tranny fluid changed, the seal replaced, that pan gasket replace, and have the neutral safety switch checked. Also, take the van to a local parts store and have them run a diagnostics test on it, which will bring up codes for anything that may be wrong with the van (engine, tranny etc). Most parts stores will do this for free and only takes about 5-10 minutes.
Once this is done, start with the cheapest and smallest parts that may need replaced. Considering you are having a leak issue, I strongly suggest having the seal replaced, which will require the tranny fluid, pan gasket and filter replaced as well, since they will have to drain the pan to pull the tranny in order to replace the seal. Have the tranny flushed, and replace the torque converter.
Since the seal and converter were not used in my van, as I scrapped a few days ago; I have them still and would be willing to give them to you at less then half the cost of what I paid for them. I have an add on craigslist for them but have had no takers for them. They were never out of the box, so they are literally "brand new".
To contact me in the event you are interested in this option, please e-mail me at email@example.com
If not that's fine too. I hope this information is helpful in fixing your van. I must say I miss my van, especially considering I paid $3000 cash for it, and it only had 89,000 miles when I bought, and 112,000 miles when I scrapped it, not to mention brand new parts all over.
Total I was out on this van including purchase price was about 3500 dollars. And when I scrapped out, I only got 374 bucks for it. Anyway, let me know how things go, and if you need any further advice or help with this.