Yes +1 ^^^
You will need to do both heads as the 4.0l OHV heads had a weak spot between the valve seats that would crack if engine overheated and since you don't know the history of the engine and it has a cylinder leak on #6 you can bet it was overheated, so both heads need to be checked.
Even if you do the work yourself you will be looking at $400 for parts and service to heads
But worthwhile since truck has alot lower valve with current condition.
My '94 4.0l is at 350k now and I did blow a hose and didn't pull over, drove it home, DUMB DUMB DUMB, lol.
Expensive lesson at 250k, but after replacing one head(cracked) and inspecting the other I have had 100k more miles without issues.
I have used Rislone Block Seal Head Gasket Fix to temporarily get by, used it on a few engines and it worked, but don't expect more than a few months.
Used it on a neighbors older(80's) Chevy V6 he drove it like that over a year before trans went and he scrapped the car, lol.