Pickup or 4runner?
From a technical standpoint, it isn't hard.
From a PITA standpoint, get ready for some profanity.
The two biggest challenges are (1) getting to the top bellhousing bolts and (2) dealing w/ lowering/lifting the transmission.
re: bellhousing bolts - get several socket extension, enough to make about 20-24". This will allow you to reach them from way behind either up under, or even inside the truck. IIRC you can take off the shifter boot/column and reach down through the hole to run the ratchet.
Also, consider removing the body bolts and jacking the body up off the frame about 2.5 inches. This will make them way easier to get to.
re: handling the tranny - a trans jack is your friend. Or, us ratcheting straps run across the frame on either side to cradle it, and slowing lower it. Actually you don't even need to totally drop it, just lower a bit and slide it back enough to get into the bellhousing.