I tried flat towing my 40 with my 80 (planned on this setup for a 10 hr drive to Rubicon). My setup at the time was OME mediums, 1.5" spring spacers and RS9000 shocks on the 80. The 40 was my daily driver/trail rig and had good castor and tracked really well on 37's. I hooked it all up and did a 10 mile test run on the highway. It got scary really fast for me. Bumps etc in the road that would cause the 40 to shift at all would transfer to the 80 and start a harmonic sway back and forth that would get increasingly violent until I slowed a LOT.
For me I think the problem was the OME medium springs were just too soft and couldn't control the movement. Needless to say I abandoned the plan to flat tow on that trip and borrowed a friend's Suburban and a trailer.
On the flip side, another local friend just flat towed his 40 to the Rubicon and back with his 80 (stock) with no trouble at all. So in my opinion you should be fine with the right setup, and stiffer springs seem to be better on the 80, so be sure to get the OME heavies. Other advice that has been given is good too.
couple pics right before my white knuckle test run.
