Yes it only happens on the highway because those speeds (over 45 MPH) and RPMs (over 2500) are required to be constant for a period of time to set the code.

This was the one I bought.
BWD Crankshaft Sensor Part No. CSS813
http://shop.advanceautoparts.com/p/b...ontent=general

 

This might be related to the Technical Service Bulletin about the plug wires being too close to the coil wire on one side of the engine causing the cylinders on that side to misfire due to induction.

Have a look at this:

http://dodgeram.info/tsb/1998/18-48-98.htm

 

I bought one of these and with in a few months it would drop out every now and then. It progressively got worse so out it went and in with the delphi. Been running great ever since. BWD=

 

Mine went within a year too (BWD) but replaced with Autozone's duralast brand sensor being in a hurry at the time, but still running 1 1/2 years so far.
next one will be Delphi for sure

 

Went through this nightmare. Read numerous threads and followed the advice down many dead end roads I changed crank sensor two times before buying a delphi sensor been two months since a check engine light. Always set a light at 52-55 mph for 20 miles or so Bought sensor at rock auto Be careful of so called experts offering advice.

 

I've had the same issue with an 98 b1500 3.9 i'm beginning to wonder if it might be an issue with the lockup converter as it always happens at 52-55 mph and at the same distance of travel. I can drive for days and if i don't exceed 50 mph no check light.Is it possible the ecm is lookin for a certain rpm when in lockup and seeing something higher because of a lockup issue in the converter and interpreting that as a misfire. Just a thought.

 

When I had those codes I had a burnt valve (confirmed) and a cracked head (suspected).

New heads fixed the issue.