Take the drive shaft off and see if there is play at the pinion shaft. Easy check.
Here is a link for axle id'ing http://www.jeepz.com/forum/drivetrai...ification.html
It could very easily be a D27 for a 70 CJ. Most get changed out for a D44 or at least a D30 due to the lack of strength on D20 and D27's.