When i watched the video, my blood ran cold, and shivers down the spine. Being a HGV Driver my self, when fully laden at 44 tonne, travelling at 56 mph, you need full concentration.
To get the retainer off the gear tower, use a small screwdriver to prise it from the clips, the bogie should drop out. The bogie is also a clip fit, but before removing the bogie frame put the retainer back on (the gear housing is in two parts).
Regards
Andrew.