You have a solid team. You have great QBs, decent RBs (I'd be starting Lynch and Spiller with MJD out), Cruz/Colston is an above-average WR combo and Turner/Cobb/Amendola are good flex options.
The only trade I'd consider is one of your QBs for an RB1, and only an RB1 - don't take anything less. If you can't find a deal like that to make I'd say just stick it out with the players you have.
Even at 3-5 you shouldn't be in panic mode and you shouldn't be thinking about giving up. Once Amendola (and eventually MJD, hopefully) come back you'll be looking better, and I'd expect you to make the playoffs.
Give my team a look? viewtopic.php?t=528273
QB: Matthew Stafford, Colin Kaepernick
RB: Ray Rice, Trent Richardson, Doug Martin, BenJarvus Green-Ellis
WR: Brandon Marshall, Marques Colston, Hakeem Nicks, Percy Harvin
TE: Aaron Hernandez, Brandon Myers
K: Lawrence Tynes