Take off the FEL, and try it again. I imagine the FEL with bucket weighs about 1000lbs, with its CG well forward of the front axle. It is probably making the rear axle light by 600-700lbs. So most of the traction is in the front wheels. The rear end will slide around and spin quite easily. The gain in traction from having the rear blade disappears as soon as it rests on the ground.
Much of my property is a hill, with parts of it 20 degrees and more. My L3200, if I am in 2WD with the loader, the rears spin a bit frequently uphill, and the brakes are quite iffy going downhill. I have to use diff-lock a lot. And I have filled rears plus 140lbs/rear wheel weights!
When I dismount the loader I take a quantum shift in traction to the rear. About a day/night difference.
My old L185DT - shown in my avatar - weighed 1800+ lbs bare. I had 2 of the rears tires filled, plus dualies, plus that roll cage (over 200 lbs), plus either 140 or 210 lbs/side rear wheel weights Total weight was up to about 2800lbs before the loader. I used it to cut my driveway, and a lot of run-off ditches for myself and several neighbors. It was pretty much like a tank, but had enough surface in the dualies that it didn't tear up the lawn.
