Well by reading everything it sounds like something in the trani. This is what i would do. take the trani out. on the bench turn the spur gear and see if the out drives turn. if so put a load on it like a pair of vice grips on each one as to stop the out drives from turning. Then spin the spur gear again and see what happens. If it will not move, or it will only move under lots of pressure which is basically defeating the slipper then your trani is fine. (one of the problem's I have had was the dog bone cups (outdrives) were slipping on the shaft becasue the screw fell out) Now if that is not the case then you have to look at the wheel's/ hubs to see if there is any slipping there. Are the Hexes on the drive shaft spinning? how i check this is to use vice grips again to hold the dogbove side of the shaft (at the wheel end) then try and spin the wheel. If there is spinning there then, presto you have found your problem. I have the Overdrive which uses the same parts and I know that the hexes on the drive shafts are friction fit (they are tapered) not using a key. these can slip if the tire is not on well enough or they are worn out. How they work is on the shaft there is a cone shaped part that the cone shaped hole in the hexes fit into. if the hex is worn too much it will just spin in the wheel and go nowhere.

Hope this helps and if you need any more clarification of this explanation please let me know