Doc, I haven't had any personal experience with them, but I've heard reports of the Poly spacers not holding up under the Cummins. I run 2 1/2" homemade (not my home) aluminum spacers in the front of my 4X4.
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'97 2500 CC 4X4 5spd Sport
'83 K5 Blazer 350 TBI (ex 6. 2), 39. 5 TSLs, 3" lift, Dana 60/GM 14 blt, Detroits