first off what condition is the fluid? Is it dark? look or smell burnt? You said old and sluggish?

Then pull the strainer assy at the base of the case and check it for debris and clean it.
Then check the inching pedal adjustment, it needs to be verified to insure it is releasing correctly.

A pressure check should be performed on the transmission controls for correct main pressure and clutch, torque converter and lubrication oil pressure.
There are 4 test plugs on the control valve that need to be removed to check for each pressure rating. The service manual has the proceedures and diagram for the control valve test ports and a pressure chart to follow. If you do not have a service manual you should be able to obtain one from your local cat dealership.

And lastly if the pressure checks test ok then the control valve needs to be removed to gain access to the clutch pack area to check the clutch plates for wear.

In most cases when i've encountered this scenario when it drives good in reverse and does not want to pull in forward(but engages and tries to pull), it has almost always been worn out forward clutch plates. When you drain the fluid and check the strainer you will most likely find alot of trash/debris. In this case the transmission is replaced with a subsidized replacement transmission. (contact your cat dealer for details on subsidized transmissions).