Possible transmission issue

So I will be driving along at different speeds sometimes the engine will rev even going around thirty five mph but the transmission will not shift and it is not only at thirty five mph it just randomly picks a speed and does it but not all the time either. I know the transmission is slipping my question is is it cheaper to have my old one rebuilt or just replaced? If replaced should I go refurbished or new? Oh also, where the drive shaft connects to the rear end makes a very loud click when shifting into gears from park to neutral to reverse to drive mostly from drive to reverse and back.