I have a 93 yukon (full size blazer)

i recently replaced the motor in it (350) about 300 miles later my service engine soon light came on and when i would go from 1st to 2nd gear it would throw me into my seat. If I wanted it to stop i would pull over turn it off and take the key out for a couple of seconds and then restart the truck and my service engine soon light would go off and would shift normal again. but now the light will not go off and shifts hard from 1st to 2nd

yes it is the original transmission with around 180,000miles

 

Also it is a 700r tranny

 

Well, the 700r4 transmission is not computer controlled except for the torque converter. If the TCC solonoid sticks on the converter will stay locked and it will act like a manual transmission when you do not press the clutch pedal. The engine will just die.

If you do in fact have a 700r4 (has a cable running from the transmission up to the throttlebody linkage? If so I would do a search for the adjustment procedure.

However since you said you can restart the engine and the problem will go away this would make me think it's an electrical issue and you have a 4L60e which is what you should have. Have you bothered to use a scanner to find out why the light is coming on?

Most likely the transmission has defaulted to high line pressure and that is what is causing your harsh shift.

Take a look through here and see if anything looks familiar.
https://blazerforum.com/forum/engine...learned-53410/

 

need to find out what the codes are