http://therangerstation.com/tech_library/4_0_Page.shtml
"The 4.0L OHV (Over Head Valve) V-6 engine found its way in to the Ford Ranger in 1990 and ultimately replaced the 2.9L V-6. This evolutionary redesign of the American 2.9L solved many of the reliability issues that plagued its predecessor. A beefier cylinder head design eliminated the 2.9L's common failure of cracked heads. Hydraulic roller lifters replaced the simple hydraulic lifters used in the 2.9L, which were sometimes overly sensitive to oil contamination, often requiring the lifters to be replaced prematurely. However, one major design fault was not completely eliminated: Valve rockers and upper pushrod tips still received poor oil supply, resulting in eventual wear to these areas, and consequential valvetrain noise as a result of the increased clearance. Required replacement of these parts is common in older engines.
The 4.0L OHV engine was produced until 2000 and was used in the Ford Explorer and Ranger. Output was 160hp and 225 ft·lbf. Though there is some variation, typically 155hp is quoted as horsepower for 1990-1992 applications.
In 1998 the 4.0L SOHC (Single Over Head Cam) engine replaced the 4.0L OHV engine producing 207hp.
We know that the Ford Ranger switched from the OHV 4.0L to the SOHC 4.0L in 2001, but the Ford Explorer wasn't as defined. The Ford Explorer actually started using the SOHC 4.0L in 1997 along with the OHV 4.0L. It wasn't until 2001 that Ford completely dropped the use of the OHV 4.0L in the Explorer. So that means that a 1997-2000 Ford Explorer can have either a OHV 4.0L or SOHC 4.0L, so you definitely need to look at the intake manifold and see how it's stamped."