The rear main oil seal would be the bigger repair job on most engines.
Remove the you joint and driveshaft. Remove the rear end seal plate. Remove the seal and bearing. Reverse the process to install the new seal.
You will have to replace the rear main seal. This will require that the transmission be removed.
Save your money, only a new seal will repair a worn out seal. Be sure the crankcase ventilation system is working properly.
Cost of a crank seal leak repair
Front (side away from transmission) is cheaper by far. Around $100.00 is my GUESS.
both front and back oil seals are big jobs have to remove timing cover and timing chain on front seal and rear seal have to remove or at least slide back the transmission to get to it so is a really big job had one done cost almost as much as pulling out an engine
Front seal: remove engine or transmission and replace the seal. Rear seal, remove the drive shaft to replace the seal. Unfortunately, you typically only have problems with the front seal.
the front or rear seal on the timing chain side
I just gotten a quote for a 2001 ranger 2wd 3.0 automatic. The quote was $550 to replace the rear main seal. This is from a privately owned repair shop.
Once you get to the seal it is an easy repair. Problem is you must remove the drive shaft, 4x4 differential, and transmission to get to the seal. Not an easy repair.
They don't make anything Liquid in a can that will FIX a rear main seal leak. Time to replace the seal and be done with it.
rear main seal is the gasket around the crank between the rear of the motor and the transmission. front and rear main seals are usual spots for oil leaks.