Transmission seems to disengage

This does sound like a governor or pressure issue. However, what you are describing is normally called a flare. That means the engine revs but does not accelerate. Try putting the transmission in a lower gear like D1, 2, or 3 and see if it still does it. If it does it could be low fluid or just the pump. However, did he do any diagnostic work to determine this was the issue?

If all he did was drive it and say that it needs a new transmission, that may be an over sell. If that is the case, I would recommend taking for a second opinion.