1997 Plymouth Voyager Torque Converter solonoid replacement

Correct, its a 3 speed transmission with a lock up torque convertor and it shifts into "overdrive" at 40-45 mph. Its not a true 4 speed, but when I am driving, it shifts from 1st to 3rd, then I use to feel a little bump when it reached 40-45 mph.
When I first drive it, if its been sitting all night, I go down the road and it shifts 1-3, then I feel a little bump, and I can hear a change in the engine rpms. Once its warmed up, I it will shift 1-3, but I dont feel that bump, and the rpms dont change.
Is it something that has to do with the turque convertor solonoid that controls that and can quit working once it warms up? As I said, it works when its cold, but once its warm it will shift fine from 1-3, but I dont feel that extra bump. Do you better understand my problem now? I am going on vacation in 2 months, and i'm driving almost 800 miles one way, and I dont want to have any problems with it.
Can this be my problem? And if so, how do I fix it?
Please help. Thanks in advance.

No "Check Engine" light is on, it came on a while ago, and shut off by itsself. My last filter and fluid change was 15k miles ago. It should have a while to go before I have to change it again right? How do I get to it? Do I have to drop the pan? I know how to do that, I just dont know if I have to remove the valve body, or if the bolt or nut that holds it on is just inside the pan. I have another transmission I can use for parts if I have to. Thanks in advance.