do the back wheels pick up off the ground when you lift a load?
are u using other employees as additional counterweight?
then id say you should make a call

remember those ratings are for a load with a load center of 24"from the face of the forks and 24" up from the top of the fork blades

you have to look at the data plate and figure out the load center of the tub

you should be trained and have the osha cert this is covered in the course
as of 2000 i believe its illegal to operate a forklift without the cert


say if the load center of the tub the way you lift it is 36"
then the rating for the machine's max load would drop significantly and would be very dangerous