I've found when removing heavy gearboxes it's a good idea to have 2 ratchet straps over the chassis rails and under the box. When removing, have the straps further back along the chassis and just barely tight so it swings back, then lower it with a trolley jack. When refitting use the trolley jack to lift it then place the straps towards the front and take the jack away and let it swing forward. A bit of adjustment with the ratchet or jack to get it right. I hope that makes sense because it works a treat.