I own a 1999 s10 balzer. 170000 miles. Lately it has a strange rattle that sounds like it is comming from the passenger side under the vehicle. The sound is there when I first start it up. But it cannot be heard while driving. If it turns out to be the flex plate how hard is it to change out. The blazer is a 4x4.

First check the flywheel to convertor bolts.

Pull engine or transmission to change it.

Engine R&R around 5.5 flate rate hrs.
Transmission R&R around 6.5 hrs flate rate.

I would be far more suspect of the catalytic converter. Climb under it when it is cold and bang on the converter with something (with the engine off). If you hear rattling in the exhaust you found your noise.