Is it easy to replace a BJ in the field? From what I recall, I had to rent a separator from AutoZone, use various long handled tools, MAPP gas torch and some cussing. I think it's more practical to swap out the BJs at home if it's a high mileage unit.
The one time I was unsure of my BJs going to fail was when I drove the 4runner to Death Valley. I simply carried two LCAs and one CV axle in the spare box. Those three things take up a LOT of room but I didn't want a broken BJ on an obstacle, 16 hours from home.
Regarding the hammer, a decent weighted hammer serves well for field repairs as well as driving tent stakes into hard ground. Just be prepare to pull them out with some kind of leverage afterwards
While I've never done this, using two equally weighted hammers can be used to drive out tie rod ends by smacking the rod end at the same time and with same velocity. Something about elastic deformation and blah blah blah. I just carry a tie rod end separator tool