You can also bolt a 4.0L OHV in place of the 2.9L without much work (if you can change an engine or clutch you can probably convert a 2.9L to a 4.0)
In 1990 the 2.9L had a mass air flow sensor making the conversion even easier........
Also as stated above the 2.9L can last a good long time as long as it never ever gets hot.......any excessive heat (bad fan, water pump, lean condition) and the head gaskets will fail.
When I go look at a used truck with a 4.0L in it, the first thing I do is flashlight all around the heads, look at where the heads meet the engine block, any green seepage? You can spot external head gasket leaks right away.
I looked at craigslist in Spokane Washington yesterday, the closest major market to me. There are SEVERAL 4.0 pushrod rangers around here for under $3000....3 or 4ea 98+ trucks too
4.0 pushrod (ohv) with M5OD manual trans and you will be happy.
2.9 or 3.0 not as happy
Craiglist drives the market in most places around the USA, you can see what Rangers are going for in your area and use that to your advantage when shopping......if I like the first truck I see but the guy does not want to move on price I will mention the other two rangers I am going to see today and as soon as you start to leave there is another chance to re negotiate the price......