2002 Dodge Caravan - Possible Tranny problem - Need help!

Hi everyone,
I have a 2002 Dodge Caravan that I just got a TON of work done to...
New Computer
New wire harness
New Front brakes
New front rotors
New front calipers
New Master cylinder
New ABS Modulator Valve

That is a loooooong story, but after all that work, now I have another problem. When in reverse, it seems as if the car has it's brake on. I can still move it in reverse with no problem, but it doesn't free roll in reverse. I was told by a trustworthy mechanic that it is a transmission problem. Can anyone give me more insight to this problem? Could it be a symptom of something else? Any input would be great.

Thanks!