Been awhile since you've had your problem so I assume you solved it... I've got the same problem with the same model 1542 tiller. I run it behind a New Holland 2120/43HP....... Tiller will spin when out of ground, but stops when lowered to ground...
The two mowers I have all have external shear pins so you can replace very easily..... I don't see an external shear pin on the tiller.... Is there one? how/where do you access? Also looking for manual on the unit... Hope it's not the drive shaft into the gear box