2 Wheel drive failure

I have a 2002 tacoma 4wd TRD.

During a recent off road excursion I somehow lost my 2wd, while 4hi and 4lo work perfectly. Everything up to the rear diff works fine (the drive shaft spins when in gear in all positions of the trans case). When in 2WD, the drive shaft does not drive the axle, however when in 4WD, 4LO and Neutral the drive shaft will engage the axle (it spins as the vehicle rolls.) I am guessing there is some problem with the rear diff lock sylinoid mechenism but I have no idea where to start. Anyone had this problem before? Any help would be appriciated!

Your post is quite confusing. Can you be more specific as to which drive shaft is doing what and when. The rear shaft does not do any engaging/disengaging.

Forgive me, forget my first post. I did some more debugging and found this:

When the rear diff lock is enabled, the drive shaft will drive the rear wheels.

When the rear diff lock is disabled, the drive shaft just spins and does not drive the rear axle.

This happens regardless of trans case position as I was able to engage the rear diff lock in 2WD and move forward. If I have it in 4WD without the diff lock enabled, it just becomes a front wheel drive vehicle.

Sounds like a broken axle shaft. With the diff unlocked all the power will go to the broken shaft. With it locked it will drive the good one.

If I'm looking to replace the passenger axle, what do I need? I located an axle for $100, whould I anticipate needing anything else? bearings, etc.?

I take it you pulled the rear end apart to verify what parts are broken?

It depends on what the new shaft comes with. If it is a used unit it may come with the bearings and retainer already on it. If not then you wll need new oil seals, and bearings.