What is wrong with my Transmition?

I have had problems with the transmission in my 2003 Dodge Intrepid, it had a seal leak in the past that i had repaired. today i was coming on to the interstate, and as i was accelerating there was a hard thud when i got around 60 to 65 mph. I immediately looked at the gages and saw my check engine light on. my car was still running and would continue to accelerate. however if it dropped below that speed and then i sped up it would do it again... could this be a problem with the Transmission control module or just the transmission in general.