seems to me like that is usually the speed that you run when you pick up tire type problems but then I dont have a 42 model truck..does seems strange that it would start there,, what is the top speed you drive it? you might slip it out of gear at that speed for a moment and see if it stops.. if it does, then I would think it would be in your drive line..

Redryder pix
My Hotrod
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