im with venom...
You said "the transmissions not be be changed since 50k miles"...
Are you refering to the tranny itself or the fluid? If its the fluid, then your tranny is cactus pal. 200,000 miles on the same fluid will destroy any transmission.
Take it to a mechanic for a proper diagnosis and weigh up your options.
What doesn't make sense is that you have had the engine replaced twice... why? also, why wasn't the tranny checked or fluid changed when the engines were put in?
Hopefully you have been changing engine oil...