Shimmy in Rear of 64 C20

[64tantruck]

I have a shimmy at about 45-50mph. Smooths bout above 55 mph +/-
Truck is a 64 frame with 78 rear 3/4 ton axle and air bags. Ive been underneath everything is tight. I have rotated wheels and had them re-balanced. Again nothing up till 40-45mph then smooths right out abouve 55mph As my kids put it "it feels like the truck is wiggling it's but"
Staying close to home most of the time dont go up in the highway much because of gearing. 65mph= 2900 RPM.
So its mostly a cruising truck, we dont race it at all.
350ci with TH350 tranny and 3:73 rear gear. 28 inch tall tires.

Thanks,
glenn