It could be caused by the welding, or it could be something totally different and just a coincidence????
How old is the tractor? Still under warrenty???
I have welded on vehichles and tractors and have never unhooked a battery either. Never had any problems. But I think my tractor has minimal computers and the tach is cable driven.
The newest thing I have welded on was a 1995 with a OBD I 5.9L dodge gasser. Again, no problems there either.
How is the tach driven on one of those. Is it a crank sensor or something? (since no spark or coil to feed tach). Might want to start at the sensor and see if you are getting any supply voltage, and see if you are getting a signal out to the instrument cluster as well. A good DVOM will come in handy.
Good Luck