It is DIY if you read the correct sources don't follow rave for god sake.
I had a gearbox on the bench out of a 4.0 Gems that had be rebuilt, only issue was it would put you through the windscreen everytime it changed gear, turned out that one of the valve and spring combinations in the valve block was backwards.
The tech had been following RAVE which i'm not surprised at, the gearbox rebuild section is a joke.
Also worth noting, your tranny is probably due a rebuild if it has been slipping for an extended period, the clutches inside are wafer thin, so any loss of line pressure will cause detrimental damage.