you should have had a front and rear diff....drive to front axle and drive to rear axle both turn same direction...so one diff has to run opposite direction to keep all four wheels turning same direction....I suspect you have your front and rear diffs swapped. I'm not very familiar with this rig, but based on the information you have given so far, this is certainly where i'd start
just a quick double check before dissassembly...rotate your spur by hand...
if viewing your rig from the back, looking front, the spur should rotate counterclockwise....opposite the direction of your clutch bell. your front and rear driveshafts i believe will turn clockwise, but not positive, it's unlikely you got a gear put in wrong to reverse out in the transmission. if all that is rotating the direction it's supposed to...then check your diffs...upside down...or front rear swap would give you the symptoms you described.
I hope someone with your rig jumps in here to verify this information....
I doubt your engine is turning counterclockwise..unless you are referring to viewing it from the front. either way, it's not likely it's running backwards.