What transmission? I have a 79 F150 4x4 with 35ö Swampers, with the NP 435, and it was doing the exact same thing about 7 years ago now, it drove smooth is first, second and forth, but third it would shake the shifter and pop out with a pretty decent metal Bang. I ended up pulling the trans and found the output Ball bearing for the trans to be completely shot (1/4" of axial play, and 3/8" of radial play). I think I spent about 150.00 bucks for a bearing and gasket kit for the transmission, while I was in there, I replaced the plastic/ Teflon guide feet on the shift forks too. The rebuild wasnÆt that bad with standard tools, the worst part inside the trans itself, was dealing with the needle bearings on the counter shaft, and getting one bearing out of a bolted on aluminum piece, I cant remember what shaft this aluminum bearing holder was for, but the local trans shop got it out for me. What transfer case do you have?

I too am in CT, but up north now, but when I rebuilt the trans, I in Idaho, so I cant help you with a good trans shop.

Sean