Sounds like your clutch plates started to slip and then warped inside the pack, essentially welding themselves together. Low pressure is what causes this, so make sure you find the cause of the low pressure and don't simply replace the clutch plates and put it all back together. If the tractor has Dual Power, be sure to check your pressures in both low and high range, with the oil at normal operating temperature.
Access is easy - through the 3-point lift cover UNLESS the tractor has Load Monitor. Then you're looking at a rear split.