Trouble on grades...

Hi all.
I just noticed something in my 96 GC and I thought I'd ask about it here. I have the unfortunate luck of renting a room in a house that has a sloped driveway. I can't guesstimate the degree because I'm no engineer, but suffice it to say if you were in a wheel chair you'd get crazy hairy speed before you hit the road, and the driveway's only 15 ft long. Anyway, what happens is, when I back in my driveway (i.e. front of van facing DOWNhill), and put the van in park, after I release and it rolls into locked position the engine starts to struggle. It almost seems like it isn't getting enough fuel.

It doesn't matter how much fuel is in the tank, it happens all the time.

Also, I know about torque-lock in the tranny and that I should use the P-Brake, but it has never been used and I service the brakes myself, therefore I'm afraid it will seize closed. Or maybe it is seized open.

I don't know if this is a big deal, just thought I'd ask.

Thanks for any help!