ive searched all over the interweb but cant seem to find anything quite similar to my problem. 96 xj, 4.0 ax15 np231. this problem just began a couple days ago. driving at lower speeds, there is no problem at all, but when i get to around 45ish (i think, speedo doesnt work) the shifter starts vibrating BAD, and this continues until my speed drops back down. it doesnt come on slow, right around 1400rpms in 4th gear (i also have 33s), as soon as i cross that threshhold, the vibes start, drop below and they go away. i can also hear a noise which the only way i can describe it is it sounds like gears meshing with no oil.
however a few weeks ago, i checked my fluid, running 10w30 in the trans and atf in the t case. i checked the t case today and was low (due to dropping a d shaft a few times, problem solved with new sye), had about 1/2qt in it, replaced with 10w30 synthetic. noise persists.

upon further testing, it seems worse in 4th gear. still does it in 3rd and 5th, but not as bad. i will hear the noise, but the vides are not as bad.
dont think its anything to do with my front end, the front d shaft is not connected. just swapped in another d35, but it was running smooth until a couple days ago. also have a home made driveshaft which ran almost vibe free despite not being balance. i have been running that shaft for about a month, previously with 36s, and never had a problem.
sorry about the long story, am i looking at a new trans in the near future? what are your thoughts? anything would help at this point

 

Bad u joint and your transmission mount is bad. Do you have an sye?

 

U joint on d shaft are only a month or so old, replaced when i made the d shaft. The mount on the trans crossmember was bran new about a year ago. And i have an AA sye about a month old.
I checked the u joints already, but ill have a look at the mount