Last year I bought an 84 fl250 that had been parked for approximately 12 years and I've been slowly replacing old parts as needed. Today something popped up that I'm unfamiliar with. I was riding it just above idle and when i went to give it hard gas the Engine accelerated properly, but the reduction case/gearbox was barely engaging at all. I let off the throttle and it seemed to wanna go, but everytime i went above half throttle, the same thing kept happening. I've spent hours online searching, but haven't really found any info. Please help!

Sounds like a couple of things it could be.
1) Is your belt worn out ?
Measure the width of the belt. I'm not sure what the width of a 250 belt is but a good way to tell is if the belt is below the top of the driven shiv when it's just sitting in your shop then it's worn out. Buy a new belt and try it. You should always have a spare anyways.
2) It's possible you sheared the key on the shaft.
You'll be splitting the gearbox to get a better look.
CO