Keith, does your 42 3/4ton have a two-piece drive shaft with a center support and rear half open???
If pista.... has a stock 1/2 ton then there is only the front u-joint to worry about.
I've had a similar vibration in my 1950, 3604. It was discussed at length in earlier threads. Right around 42 or 43mph I get a vibration, feels like the rear end is hopping up and down, at around 46 or 47mph it goes away. All new and I mean new driveline, three new u-joints (in phase), a new center support brg. and rubber. A new torque tube bushing in front. The rear end was disassembled checked for wear and set up according to the Shop Manual, the axles were checked for straightness and indicated for run out at the lugs and flanges, all ok. New front and rear motor mounts were installed as soon as I brought the truck home.
I've had the original 15" bias ply tires, meticulously balanced. Then replaced them on the original two piece rims stripped bare and re-sprayed with PPG DBI, with brand new Power King bias plys with new flaps and tubes and meticulously balanced on three different visits to the tire shop. Then replaced those with later 16.5 tubeless rims and Cooper tubeless radials and had them balanced at two different shops. \
With the truck up on jack stands in the garage and run up to and through the troublesome speed range there is no vibration.
All of this and, "I still get a vibration at right around 42 or 43mph that feels like the rear end is hopping up and down, at around 46 or 47mph it goes away."
At this point I'm of the opinion that there is some sort of harmonic vibration set up at that speed as the chassis flexes. If you accelerate steadily up to 55mph you don’t notice the vibration, but if you hold it at the low 40’s for a few seconds it starts to vibrate.
Good luck!!!
Denny Graham
Sandwich, IL
