Go to Harbor Freight or Amazon and get one of those cheap magnetic angle finders, round dial with red needle. Its the fastest way to find driveline angles.
All you need to measure is the transmission shaft output angle to the driveshaft angle. Then driveshaft to pinion angle. Both ends need to be in the 1/2 degree to 3 degrees range for ideal operation and close to equal. You can be up to 4 or 5 degrees, but it must be the same at each end. The rear angle is always changing due to bumps, humps, loads, and general driving around, but as it changes, the front is also changing, so equal them them out. Starting at the ideal 3 degrees insures that during most of your diving, the u-joints have roller bearing movement and are not at the extremes of there operating range.
Here is what a phased and out of phased driveling look likes,
https://managedmobile.com/wp-content/uploads/2018/02/driveshaft01.jpg If the guy that shortened the driveshaft welded the cap back on wrong, it would explain your issues. Even a few degrees off would be wrong. A two piece driveshaft would have the second section 90 degrees from the first.
With a slower driveline rpm, balancing will not be as much of an issue. It's still important, just not as much as would be with higher gear ratio.
And with the lower gear ratio, the engine and driveline has a lot more torque on it trying to pull the same load at a much lower rpm.
