There are TONS of arguments for doing a LS swap and I'll agree with some of them. But for ease of doing plus easy on the pocketbook a Dodge V-8 magnum engine is your best bang for the buck. They are old style push rod engines but were one of the first mass produced roller cam engines. Nope they don't make as much power as a LS does but honestly how much do you need in a TJ?
Are you wanting to go automatic or manual? One BIG benefit of doing a magnum swap is if you use a 5.2 or 5.9 from a Jeep and keep the automatic. This puts the starter on the passenger side.
I would tell you NOT to just take your Jeep to any Speed Shop. They don't know or understand Jeeps/4x4's/Off-Roading as a whole and you want build around this which isn't what a speed shop does.
There are enough thread on here for all 3 popular swaps.