From the description of the wheel hopping I'd suspect your sprag was engaging, at least partially. My experience has been that the sprag will only engage when the rear wheels are slipping. It will disengage as soon as the rears grip and you let off the power.
To test the sprag:
Jack either front wheel up. Be careful as the wheel starts to rise. If there is any torque built up (called "sprag wind-up") the wheel can spin violently as the tire breaks contact with the ground. It can be especially spectacular if you leave a lug wrench and handle hanging on a lug nut.
With the transmission in reverse the wheel should turn backwards but only turn a very short way forward before stopping. With the transmission in first gear the wheel should turn forwards but only rotate a short way rearward.
If this were an M809 series 5-ton truck (M813, M818 etc) the procedure would be the same except you would need the air system pressurized.
Let us know.....
Lance