Your problem can likely be solved by simply changing your retrival technique
at the ramp rather than making significant alterations to the boat or
trailer. Also, rather than trying to determine the exact amount of friction
involved between the boat and the trailer bunks (how many fairies can dance
on the head of a pin?), simply accept that friction is there, it is
significant, and put it to work for you instead of against you, as follows:

Your problem is simply that you are backing your trailer to far into the
water when you retrieve your boat. Since you don't have the ability to
winch your boat to the bow stop, you are instead floating the boat onto the
trailer, up to the bow stop. This works fine until you start to pull the
boat out of the water, when the boat will naturaly settle back a few inches
back on the trailer. Reason? Not enough friction between the boat and
trailer during those first feet of pulling the boat and trailer up the ramp
to counter the effects of gravity and inertia. I have the exact same
problem producing the exact sam results with my boat, but for a somewhat
different reason. My boat has a bow eye and the trailer has a winch.
However the boat alone weighs 12,000 pounds, and there's no way I'll ever
winch the boat across the trailer bunks unless the boat is pretty much
floating free. And if the boat is floating free, then I'll have the same
problem as you when retrieving due to the reasons described above.

The solution is quite simple. Don't back the trailer so far into the water.
Keeping the trailer slightly more out of the water will increase the
friction between the boat and trailer, enough so that the boat will retain
its position on the trailer when pulling the trailer out of the ramp. In
other words, use the force of friction to your advantage instead of trying
to analyze a way to measure it and defeat it. So how do you overcome the
friction to get the boat on to the trailer? Simple, just use the device
that pushed you around the water all day (lots of friction there) and got
you back to the ramp in the first place. Use the engine to power the boat
to the bow stop. It is more than adequate to overcome the friction of the
boat sliding accross the carpeted bunks of a trailer that has been properly
positioned on the ramp, and requires no modification of the boat or trailer,
only modification to the skipper's mindset. Power on, dude.