The transmission has been around for years in rangers and explorers (should be the 5r55e). Yes they can have problems, but so can anything. The 2-3 shift flare is really common. Basically when upshift from second to third, the engine may rev up a bit and then jump into third. A big killer of the transmission is heat too, so a good cooler will help.
The 4x4 system is pretty decent. There are no lock out hubs, no vacuum actuated anything. The normal problem is the electric shift motor. Usually if it doesn't work right it can be removed and cleaned.
Have a good look at the balljoints and sway bar endlinks and bushings in the front end, they are fairly common issues also.
All in all, I've been very happy with my sport trac so far. Zero issues (except for a few that I created myself).