GPS (Getrag preserva..) long sorry:(

[BJMarshall]

I am soon going to put a clutch in the truck and I guess I better look at the Getrag while I am in there. I am hoping to get a little help figuring out what to look for before I get it out. I have the manual for it so I can't imagine it being that much trouble. I have researched putting in a NV4500, but that will cost approximately what I paid for the truck. My truck is not overly powerful, and I don't abuse it, so I feel like I should be able to get another 100k out of the transmission.
Right now, it works OK. It shifts good with no grinding and little effort. I have been running it with mobil1 5w30 overfilled, and so did the previous owner. Last weekend when I changed the oil, I switched to 15w50 to see if that would help. It still shifts good. It's a little stiff when it's cold, but after about 5 miles, it's fine. It is a good bit quieter. The only problem I have is a terrible noise in 4th gear. Anything below 40mph in 4th and it vibrates the crap out of the truck under acceleration and it's very loud. It starts getting quieter about 40 and is completely gone by 50. It just doesn't feel right to wind 3rd out that far, but if I do, the noise is non existant. I have been revving it in 3rd to keep it from doing this, because I'm sure it is not good for it.
Any suggestions on what to look for when I get it apart? Also, any input on where to get parts would be appreciated.