Basic 3D model (OpenSCAD) for making typical pinion or caster angle correction shims for a leaf spring vehicle. User can specify width, length, thickness, angle, center hole size and also type of mounting.

First photo is an example of a typical early model Jeep shim with 1-3/4" width, 4-1/8" length, 5/16" center hole and in this case 4 degree angle.

This model file would serve 2 main purposes:

First, would be to make a 3D printed shim, perhaps to try out varying angles with the vehicle stationary, in order to correct u-joint angle(s). Then, once a particular shim design is worked out, that 3D printed part could be used as a sample to machine a shim out of a more durable material like steel. You DO NOT want to run a 3D printed plastic shim while driving, that isn't safe.

Secondly, you could export a STEP or IGES file and upload that to an on-line machine shop for a quote on producing that part. Or, if you have a local machine shop or know someone that has access to a manual or CNC mill and metal cutting saw, that STEP or IGES file or sample part could be used to duplicate the shim. See:
https://forum.lulzbot.com/t/tip-converting-openscad-files-easily-to-step-with-freecad/228

If you need to road test the vehicle, one option is to use some pieces of steel flat bar to simulate the shim. Let's say your shim tapers 3/8" from thick to thin end. You could cut a piece of 3/8" flat bar and a piece of 3/16" flat bar. Drill a center bolt hole in the thinner piece and place it in the center of the spring perch. Put the thicker piece on the end of the perch and tack weld both pieces to the perch, then clamp the spring down with the u-bolts. This way you could do a road test to see if that angle will work. If so, note the angle change and proceed to making your shims. If it didn't fix your vibration issue, adjust the angle with thicker of thinner pieces of steel.

Here's a handy angle jig that you can 3D print to use to set a precise angle on a shim in a milling machine vise:
https://www.thingiverse.com/thing:7098076
This allows you to clean up a saw cut face, make a pair of shims to the same (of different) thickness and change the angle of an existing shim.