Check Engine Light GMC Jimmy 1997?

The service engine light came on and the transmission code showed an 1870 transmission problem. Has anybody had trouble with this code before and what was your solution? Will a valve body replacement kit repair this or does the whole transmission need to be replaced. We have a hard shift between first and second gear.

Other than this the car is in great condition, very low mileage and body is immaculate. We just want it right before our son drives it in February.