How hard us it to replace the rear pinion seal on 2014 ATS at home??

Looks pretty straightforward. I haven't done one personally on an ATS, but it looks about the same as many of them.

There'* nothing special about it.

Yeah I always mark and measure, then count turns of the nut, then when going back on I make sure to turn the yoke a turn forward then backward after every turn of the nut once it'* under tension. There'* no accurate way to measure the original torque or rotational resistance in a useful way because these variables are guaranteed to be slightly different when going back together.

The alternative is to replace the bearings and races.