Have a 04trailblazer ls 2wd transmission won't shift ,I replaced the transmission at Xmas time, well going down interstate doing about 85 on cruise and rpms went up and was like I put it in neutral replaced with a used on ,now a week ago it did the exact same thing so I went and purchased another tranny and have it in but wont move, I changed the fluid and filter but still nothing ,says any and all help would be greatly appreciated,could it be an electrical issue ? Thank you in advance for the help