Motor swaps are a lot of work. Keep that in mind. If you don't have the tools or know how(or know someone that does), it could prove to be an expensive waste.
Toyota's are surprisingly reliable. Are you having problems with it right now? If not - keep it and just drive it into the ground. If so, weigh the options. Would a motor swap/replacement cost more than you are paying in maintenance?
I sold my Cherokee a few months back and while I miss it, I don't miss the maintenance. My car that I bought has paid for itself in the parts I would have saved. Back on topic though.
Toyota did make/does make diesel motors. Good luck finding one though. My suggestion would be to find a donor truck.