First thing is a scolding...Our little Jeep trannies and engines were never intended to be the brake on the vehicle.. These ain't diesel 16 wheelers. Try not to use the engine as a brake unless you are off road and crawling.

BUT your problem appears to me to be in the clutch area. I have seen worn clutch bearings in the back end of the crank cause vibration and shake. As well as flywheels and the clutch itself getting out of balance.
For myself I'm doubting the tranny is the problem..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist