darkshadowgray

So I have a 2003 mustang V6 Automatic, I daily drive it and its never failed me. I'm pretty skilled when it comes to automotive stuff but I'm having an issue diagnosing this problem. I've begun to notice when coasting at highway speeds (40-60 mph) the car would have a very low toned and subtle rumble/vibration along with the feeling of engine braking like in a manual trans. I have a suspicion that its torque converter shudder. The trans fluid has been changed (to the best of my ability, only got about 5qts) and is in a very light brown color, like dirty but not burnt. I'm hoping the case is that my torque converter needs a replacement and not my whole trans. Anyone with any idea of what the problem could be please let me know. The car has 155k miles. I've done a GT 8.8 differential swap with a 3.73 gear ratio. I planed on getting a high stall converter anyways. Another thing to mention is when I put it into any drive gear the car lurches a little bit. You can feel the whole car move as Its put into drive/reverse. It feels stable and very smooth when in park or neutral with the engine on but as soon as Its put in drive it vibrates the whole vehicle until you start moving . Any help is appreciated, Thanks!