transmission not shifting properly

I have a 1997 Chev Van 2500, 350 engine.
I have a Mass or Volume Air Flow Circuit Low Input P0102 error. Hopefully replacing the MAF sensor will cure the rough idle and sluggish starts. Just
mentioning it in case it is related to the following.
However at the same time I also seem to have transmission prolems. In "D" I have first and a hard changing second, but the engine revs wildly between second and third. This seems to have happened over the past couple of days during some heavy rain. In "1" the transmission works fine in first, In "2" the transmission works fine in second. However in "3" the transmission does not go into third gear.
The speedometer seems to register only a fraction of the curent speed. For example at 50kph it registers 20kph and can't seem to make up its mind to stay in 3rd gear.
Somegarages say it is the speed sensor, any opinions, if yes how hard is this to change on a Chev Van?
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