1995 Plymouth Grand Voyager LIMP MODE

Short story: my van seems to go into limp mode after driving short distances.

Long story: Just bought this van earlier this year and we knew that there were a couple things we needed to work on; the most important was changing one of the valve cover gaskets since it was leaking. Me and my mom ended up spending this past Saturday getting that done. Afterwards, me and my sister went for a ride to make sure everything was all right. Everything went well for miles, but the van seemed to be hesitating once we starting getting closer to the house. It had gotten really bad when we came to an incline in the road. I was pressing the accelerator, and the RPM was going up, but acceleration was was not. Once back home, we checked the oil and gasket to make sure we didn't mess up somewhere. Everything looked fine. My mom asked me to check the transmission fluid, and, sure enough, it was low. Filled it up and tried again. Same problem. Then we decided to change the filter and fluid. There was a little sludge on the magnet in the transmission pan, no problems other than that. Put the new gasket on, closed it up, and put some more fluid in (and some Seafoam Trans Tune). SAME PROBLEM!

Info:
Speedometer is showing a speed, but speed won't go any higher than 40 MPH.
RPM goes up as I press the accelerator. Will go as high as 6, but no further acceleration.
Once the van slows down, it seems like it wants to go faster; speed goes up VERY slowly.
Oil level and trans fluid level is good.
All connections and vacuum lines seem good.
Transmission itself is fine.
Will not shift out of lower gear.

The strange thing is that once I start the van, it seems to run fine. It just starts acting up after driving a short distance (about a mile). And, even if the van is having troubles going forward, the reverse is working fine at all times.

If anyone has any hints, tips, or suggestions, it'd be much appreciated! Please be specific!

Are you getting a check engine light and have you checked for codes? Type engine and transmission? It's not shifting past 2nd gear correct? Limp mode limits you to 1st, 2nd and reverse gears. You used ATF+4 fluid correct?

There's no check engine light. Checked for a code yesterday morning: #1255. Has a 3.3L 6-cylinder engine; automatic transmission. It wasn't shifting past 2nd gear and reverse was also working fine. Not sure about the fluid. Used standard automatic trans fluid from Walmart.

We changed some of the sensors on Sunday; speed sensors (input and output), neutral safety switch, and driver gear position switch. Didn't make any difference.

However, there has been a change. Checked some of the connections yesterday evening. There was an oil leak while changing the front valve cover gasket and we think some oil got on a connection somewhere and is causing it to short out. Drove well yesterday evening, even making it to 4th gear. Planning on checking wiring this evening.

Had read that code 12 has something to do with "Memory Standby Power Recently Lost" or if you have recently had the battery disconnected, which we had done before checking for codes.