first the check engine light comes on occasionally and then goes off

1999 Jeep Cherokee

I got a new(ish) Jeep over the weekend, previous owner beat the snot out of it and now I'm fixing a lot of small problems, got the list down to a few now, first the check engine light comes on occasionally and then goes off for awhile, I tried the flipping of the key and that didn't show any codes or anything then found the trip odometer trick for the newer jeeps and it didn't give any either so then I took it to the parts store and used their scanner and found p0505 and p0743. I found the p0505 is idle air control? and p0743 has something to do with the torque converter? any help with the next step would be appreciated!!! its a 1999 Jeep Cherokee classic(not grand) 4.0 auto 4wd with 225k on it... it was towed behind a motor home so who knows how many miles are really on it

Response:
How does the Jeep seem to run other than the codes? For example, does the idle seem normal? Does it do anything unusual, especially when the light is coming on?

2007 Jeep Wrangler X Unlimited and it is a 4x2

2007 Jeep Wrangler X Unlimited

I purchased a 2007 Jeep Wrangler X Unlimited and it is a 4x2. a Mechanic friend of mine was going to assist in converting it into a 4x4. Short story, we have fallen out and I have no knowledge of completing this job. My question is would this conversion be as simple as swamping out the front axle with Dana 44 purchased from MOPAR? Please advise on specific parts I would need to complete the Job