Questions about Shock Sizing

So, as the description says, I have a few questions about rear shock sizing and replacements. For starters, my bike has a 170mm, 1500lbs spring shock, and for me (I'm 6'0 and 176lbs) it's a bit too stiff to where it just feels like a hardtail. So, I was wondering if anyone can help me with this...

  1. Can I put a longer shock, like a 180mm, instead? 170mm is a struggle to find.

  2. If there are replacements for 170mm, can anyone tell me which ones?

  3. If I swap the 170mm for a 180mm, can I also increase the size of the front fork?

  4. What difference does the lbs make?

(Side note, I'm usually only able to get equipment from Amazon.)