V8 3.6 vibrations in transmission

[Denis]

My friend s V8 1991 210 000km.

The rear diff was very noisy and it s been changed. 
The tranny does kinda braking 1 or 2 secondes then after couple sec it does it again. It s like the car pull heavy weigth intermitent.

All  speeds work perferctly in any way. But still that grounding noise feels like braking.

What is that ?

It seems it s not the solenoide for change speed but some solenoide that decide to use 4x4 or not !!

any tips ?

Ðenis