I don't think they are related, I've seen other trucks with duff speedo's and its not set the tranny fault light (the flashing OD light) off, but may be wrong.
First up I'd check all the plugs on the gearbox, and the speed sender unit on the back of the transfer case. You can get the fault codes for the gearbox in the same way as the engine, Do a quick search for it, its been covered before.
The gearbox may just need a good flush, if its sludgy, things won''t select properly.
The speedo will have been removed to do the MPH conversion, could just be not put back properly, I'd check the facia unit for loose wires and plugs.
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