Ok, so today I had a chance to do most of the tests. The only one I didn't get to do was 'drive' the truck on jack stands. I figure this would be best to do with someone else's help so I can inspect while someone else drives it. Anyway, here are the tests I did get to do and their conclusions.
-Rear driveshaft removed, hubs locked and driving in 4WD (FWD). Not much difference, it was hard to say for sure because it drove quite differently like this. The issue still seemed to be there around the same 40-50 range though.
-Clocking the driveshaft on the t-case flange. Rotating the driveshaft 1/4 turn or one bolt hole on the t-case flange changed the shudder a little. It was noticeably less shaky, not perfect but a noticeable improvement. Rotating it another set of bolt holes, now 1/2 turn from stock position, felt similar to the original position. Rotating it to 3/4 of a turn or one final set of bolt holes, it felt about the same again as stock or 1/2 turn. So I ended up leaving it in the 1/4 turn position because it did at least seem to make some improvement.
-While I was under the truck I was looking at the transfer case dampener and remembered reading how some Rangers came without them and some came with them and no one really knows what Ford's reasoning was. So I figured why not try removing it and seeing what happens? Well it was definitely noticeably worse, so needless to say I definitely need that dampener.
-I loosened the motor and trans mounts, then ran the engine for a minute or two revving it up and holding it at different rpm's to get it to move around and settle out. Then re-torqued the mounts. This made little to no difference as far as I could tell.
-I double checked the angles on the spring pads on the rear axle since it was mentioned that if they are off from each other that it could cause a vibration if they weren't perfect. They were pretty much dead on, at worst maybe 1/4 of a degree off from each other. But I have a couple different sets of pinion angle shims so I tried using a 1 degree shim only on one side to see if it helped. I installed it on the drivers side, both directions at different times, so that it would point that side up or down one degree. Both of those tests made the problem worse, the shudder now started sooner around 35 or so and up to 50 as before but noticeably worse. So I believe that is not my issue.
-Since I have a couple sets of pinion shims I tried different angles to see what happened. As mentioned before, setting it to perfectly equal and opposite the t case flange, or 0 degrees pinion angle, the vibration is quite bad. I used a set of 1 degree shims to angle it down 1 degree, and it does help a little. I also have a set of 2.5 degree shims so I swapped out for those instead, and it made no difference compared to the 1 degree of pinion angle. Better than 0 but still shaky.
-Also, I recently reverse leveled my truck with a shackle flip first of all because I had the parts and wanted to get rid of the lean BUT also because I wanted to see what it would do given to the driveshaft operating angle and pinion angle would change. The driveshaft angle was reduced, it went from something like 7 degrees down to around 4. (The t-case flange originally pointed 3 degrees down, the driveshaft measured to be 10 degrees, so 10-3 equals 7 degree operating angle unless I'm misunderstanding that). This also changed the pinion angle to a super dramatic 6 degrees nose down compared to the t-case flange. I figured why not try driving it like this even though I believe 6 degrees is way too much. And surprisingly, this was the smoothest the truck has been yet. Not perfect but much better. Because 6 degrees seems excessive, I tried installing the 2.5 degree shims again, this time to bring the pinion back up so it now has 3.5 degrees of pinion angle. However, it feels about the same as having 1 or 2.5 degrees down. 6 degrees down was the smoothest, although not perfect.
So I haven't fixed the problem yet but have found ways to at least tone it down a few notches. But, the weirdest part to me is that having 6 degrees down was the best result so far. Am I mistaken that 6 degrees down is way too much pinion angle? Or better yet, does having that much angle tell of some other issue that having the excess angle masks/remedies somehow? To me most of these tests were rather inconclusive but I think that one part is the most interesting to me.
Here are some pictures of the driveshaft and angles and all that, it's hard to picture under there because either the gas tank or the exhaust block the view.
Looking at the rear from the drivers side:
Looking at the front from the drivers side: (don't mind the blob of grease, whenever I do a sloppy job greasing the u joints it flings everywhere lol)
Looking at the rear from the passenger side:
Looking at the front from the passenger side:
A shot of the whole shaft: (or as much of it as I could get)
Angle finder on the t-case flange: (reads about 4 degrees down)
Angle finder on the pinion flange: (reads about 1 degree up, giving around 3 degrees of pinion angle currently)
Angle finder on the shaft itself: (reads around 8 degrees, making the operating angle 4 degrees unless I'm misunderstanding that)
Normally I remove the driveshaft to measure the angles to be more accurate but you still get the point for the purpose of these pictures. Any thoughts so far?