Don't know if you field dress them. I do, saves a lot of weight. Can you lower your tailgate past horizontal? That helps.

If I have a big deer, and I've worked at getting it to the truck, I try to drag to deer onto a slight rise that I can back up to, so the truck is as low in relation to the deer as possible. If you can do that, it is easier than digging trenches for your tires.

I bought a cart for this year, the plywood ramp idea sounds good. I might double up a 3/4" sheet and cut so it is 2 ft wide x 6 ft long.