All signs point to a cracked head or block on my truck after replacing the gaskets. It has about 175,000 miles on it, and I'm looking to get the engine back in decent shape. Any suggestions on 1) how to be sure the bottom end is good and 2) what would be a good idea to replace preventatively if I just put new heads on it?

 

Machine shop can pressure test heads, and the 4.0l heads will show visible crack between valve seats, but if you don't see that then take heads to machine shop to have them pressure tested and surfaced if they pass, shop will also install valve guide seals if you don't have a valve spring compressor, you ALWAYS put in new valve guide seals
Usually this runs $75-$85 per head, if both pass, if one doesn't then its just a few dollars for test

New heads would come with new valves and seals, and will be surfaced already

4.0l crack looks like this: https://www.therangerstation.com/for...8-1-jpg.50931/

Cracked block would be highly unlikely, never even read about one in a 4.0l

Was there coolant in the oil?
And if so how long was the engine running with that condition?
Problem with coolant/water in the oil is that it boils/steams at 212degF, and when it gets on the bearings it can "steam clean" them, washing off oil coating
Now there is always some water in the oil, from condensation, and its burned off pretty quickly every time engine warms up, so its not a big issue
But if there is alot of water and you drive it that way for say 500miles it can cause bearing damage over time

But outside of that the bottom end should be good past 400k miles
If there was coolant in the oil then after the heads are re-installed, put in a cheap oil filter and 30w oil, then run that for first 100miles, or so, then drain and put in a good filter and your favorite oil

On the 4.0l you can put in new lifters, you can only do that with heads off, but up to you, they can get "ticky" after 200k miles, doesn't hurt anything just noisy

You could also have fuel injectors rebuilt, or swapped for rebuilt, several place rebuild them usually $20-$25 each

 

Replaced head gaskets already, but the machine shop only visually checked the heads, no pressure test. I'm hoping they missed a crack and it isn't the block.

Haven't noticed coolant in the oil, but I am seeing an oily sheen on coolant as I was flushing when trying to find out why I'm overheating.

 

Do the glove test
You can even find out which cylinder has the leak

And its very easy to do

Search on the site for glove test I have posted it several times

 

Assuming I understand the test correctly, the glove definitely moved when starting with plugs disconnected.

Guess it’s time to tear it back down and find a shop that will actually magnaflux or vacuum test

 

Did the shop at least resurface the heads ?

 

You can take the spark plugs out, one at a time, and crank engine again, when glove stops bouncing the leaking cylinder is the last spark plug removed, if it just bounces less then you have more than one leaking cylinder..........keep taking out spark plugs

This is not required but can be useful information