1998 Tacoma 4wd suspension is too soft

I just bought a 1998 Tacoma v6 SR5 TRD excab with 112k miles. I'm having trouble figuring out why the suspension on my truck is so soft and even dangerously unstable when driving at highway speeds. Is it my shocks? When I hit a bump on the highway, 65-75mph, it rocks side to side really bad. But at lower speeds it takes bumps very good almost like a Cadillac. It has Monrore sensatrac shocks at all four corners, so I figure they are not stock. Do you think I have the wrong shocks on it? it doesn't sag at all so I doubt the springs are worn out at 112k. Any suggestions?

Thanks, Chad

Monroe shocks would not be OEM.
Do you have a sway bar?

Sorry, I never got a notification of a reply. No sway bar. I also noticed that the spring shackles on the rear are at a very low angle. Id say 30* to the frame. Almost touching the receiver hitch and probably bottoms out when loaded. Haven't checked it yet. I definately need new shocks but now I'm concerned about the weak springs. can someone with a similar year Taco look at theirs and check the shackle angle?

Thanks, Chad