OBS ZF5 weird vibration issue
So my truck was driving smooth up until a couple days ago and I haven't been able to find any write ups that had similar issues as me. First off my truck leaks transmission fluid, but it has been leaking since the day after i got it rebuilt so I don't think that's what is causing the issue. Anyways, my symptoms are hard vibration felt in the stick as soon as i hit 50 mph and above, but only feel the whole truck vibrating hard when in neutral going 50 mph and above. If i'm going under 50, there is no vibration felt in the truck or stick at all. If the truck is in gear going over 50 the vibration ranges from very light to non existent besides the stick itself vibrating, but again very hard vibration if in neutral going over 50. Has anyone had similar problems, and what did they do to fix it?
97 4x4 F350 XLT OBS Powerstroke CCSB- ZF-5 manual trans, 6637 cold air intake, TS6 chip, straight piped 4 inch turboback pretire side exit, 4 pillar gauge w/ diff temp, EGT, boost, and HPOP pressure, ATS diff cover, dual alpine 10'' subs w/ alpine component speakers, B code rear springs