Thanks Henry10...
I did resolve my transmission issue. It turned out not to be the transmission after all. This is what I did: went to Transmission Repair place. They suggested a $50 assessment, but before I took that step, I went to my trusted car repair specialist that has done work on my car in the past. He said that there could just be engine operation issues that might be causing this poor performance. Engine Light was on, but always is in the Venture! After his evaluation, we opted for an EGR valve replacement, as well as fuel filter, air filter, and spark plugs. Total Cost $650 (a little high, but they have to tilt the dumb engine forward to change plugs....and I don't usually do that myself). Better than a rebuilt Transmission at $2000.
Car basically got a great tune up, and works perfectly now. Engine runs much better, and smoother, without any roughness. High-speed downshifting is great, and I guess this was because the exhaust recirculation got fixed with the EGR replacement (indicated by codes in the Check Engine Light function).
Thanks for taking the time to educate me on the Transmission.
thx. greg