Sometimes, when I'm cruising at like 40-45mph, I switch into OD. I can't remember off the top of my head, but this is the TKO-600 with the .6x OD. Anyway, this and my 4.10 gears, I think I'm somewhere like 1,000-1,500 RPM when I'm in OD at 40mph. It kind of feels like the engine is wanting to break up like it was at high RPMs or something, like it's kind of strugglin to drive or die. Should I maybe not be in OD at 40mph, and just stay in 4th? Would this cause what's happening to me? Or maybe noone has this problem, and I should look elsewhere...thanks.

I never shift my 200 4r into OD unless I'm out of the city or on the interstate. Once up to speed, it shifts into od on it's own around 55 or 60. Sounds like you're "lugging" yours down quite a bit. Probably not wise.