8.8 swap

What I have is not an A body, its a 87 Dakota.
If posting here is out of line, please tell me and I will delete the post.
I pulled the 8.25 with 10in drum brakes out and put a 2000-01 explorer 8.8 rear in the truck.
Everything seems to be working well.
The brake pedal is up and seems to be very solid.
On dry roads the truck seems to stop well. In fact, it seems better then with the drums.
The other day, I took it out on a wet road, and wanted to see how easy it would lock up and slide.
It was hard to even get the truck to slide on a wet road.
In my case, I dont believe I am getting enough pressure to the rear.
The only thing I see that would cause that is the factory proportioning valve.
The front calipers use a piston that is 2.36 in. and the explorer rear uses a piston that is 1.89 in.
So if anything, with a smaller piston on the rear, I would have suspected it would lock up much quicker then the front.
The brakes are power assist and the master cylinder is .944 in bore.
Any input would be appreciated.

And again, if this post is out of line, I would be glad to delete.