2004 Envoy SLT seat

Just found this form and this is my first post. I purchased a 2004 Envoy SLT last year b/c it was just what I needed and frankly I liked it. I purchased the top of the line SLT b/c I drive a lot. After driving for 35,000 miles I find the seats really uncomfortable. The lumbar (spelling) won't recline out all the way and there is not side support. The dealer tells me there are no problems and they can't do anything. Can I replace with an aftermarket? Any thoughts?

Kevin