An LI should be slow enough that even at full throttle the speed is nothing more than a calm window-browsing walk in low. If it takes off and you have to almost run to keep up with it, then there is something wrong with the hi-lo planetary. To access the hi-lo planetary, remove the engine.
As to what is wrong with the planetary, it could be a broken gear, crankshaft seized in the pilot bushing, or other similar things. Basically the planetary is locked up.
I'll bet that engaging the PTO is difficult too. With the planetary locked and the HI-LO in neutral that attachment wants to spin at normal speed.