Need some help with my 1969 C-30

[special-K]

You say you replaced the rear axle,but what is the condition of the Dana 70? Are all the wheel bearings good,axle shafts straight,drums not warped,housing straight out of the center housing,spindles straight and not overly worn? That's where my mind went from reading your post. That wheel hopping has got to be something "at" the wheel.
On my '92 K3500,that I bought new,I was wearing rear tires out (outer tread) in 8,000 miles and had chronic axle seal leaks. I never had any wheel hop,though. I had a four wheel thrust alignment done and found the rear tires were towed in about 3/8" on each side. I figured the axle was assembled improperly from the factory. When I finally took the axle out to replace I found the axle tube had never been welded at the center housing and could be moved all over the place,once free. The spring was holding the axle tube in place. Actually,the spring hanger was twisted a bit making the spring sit cocked from lateral force. This probably isn't your problem,but you can see that it would be a good idea to check out that axle real thoroughly if you haven't already.