We just came through there a couple of weeks ago (we were in Breckenridge for Spring Break). You will find the steepest (up) grade from Dillon to the tunnel, it is pretty much downhill from the tunnel to Denver (6-7%). We kept our toad (~4,000 lbs.) attached without any problems. Uphill we just got in the truck/hill lane and put the hazards on, slow and steady.
We did about 25-30 MPH uphill on the steepest grades and downshifted to keep the downhill speed to 45-50 MPH.
Have a safe trip!!
2009 Mandalay Presidio 38D
2009 Jeep Wrangler Unlimited Rubicon (Toad)