Welcome to the forum
1999 Ranger will have the 4.0l OHV engine, very reliable engine, my 1994 is close to 350k with no issues.
2001 Ranger got the 4.0l SOHC engine no relation to the OHV, has more power but also a few problems.
1999 ranger 4.0l will have the 4R55E 4-speed automatic, heavier duty version of the 4R44E that came in Rangers with 3.0l and 2.5l engines.
4R55E was no more problem prone than any other automatic, adding computer control added another layer of possible problems, for all car makers, like power windows and AC, just more things that can break, lol.
Biggest killer of automatics has always been heat, stock radiator cooler is not enough on most car and all trucks, so add a second cooler, best $60-$100 you will ever spend.
Google: automatic transmission fluid temperature images
Simply going up a long grade causes engine temp to go up, and transmission fluid temp as well, so radiator heats up and there goes trans cooling, lol, every minute of trans fluid temp above 195degF just ticks off the transmissions life clock, and the higher it goes the faster the clock starts to spin
You have no history on the trans, or engine for that matter, I am sure seller is honest but..............
Smell the fluid, and look at the color, that's about all you can do
4R55E valve body gasket was an issue for a few years, new gasket is bonded to valve body, and repair can be done with trains in the vehicle.
130k is getting up there for an automatic, but if fluid is kept under 200degree 200k+ wouldn't be unusual.
1999 Ranger 4x4 may also have Pulse Vacuum Hubs(PVH), Rangers 1998-2000, these were a bad idea, problem prone even when new.
Best option is to change to Manual Locking hubs, if not already done.
Ford was putting Live Axle on some 1999 Rangers, so you may have that, front axle always locked, no hubs.
Last edited by RonD; 07-09-2016 at 12:56 PM.