2008 silverado aftermarket posi

[KEL]

Hello
i have a 2008 Silverado 4 wheel drive.
It could get stuck on wet pavement.
In four wheel drive, the wheel with the lesser amount of traction on each axel gets the power.
Question is :
Will an aftermarket posi unit fit in my factory differential case, or do I have to swap out the entire rear end ?
If one will fit in, which one ?

Thanks for the response(s).