You want to get the vehicle drive shaft (torque tube) checked for any loose end play... paying particular attention to the "U-joint" area.
As bad as you are describing it it sounds like you are really close to dropping your drive shaft...
DO NOT work on this yourself if you are not TOTALLY familiar with good jacks... jack stands... wheel chocks and have a level work area available... also keep in mind that while your TRUCK may be in PARK... that is only as stable as your drive-shaft.
Don't get between your truck and a hard place until you get that DRIVE-SHAFT fixed...
Another hot tip... if you are going to the trouble of removing the drive shaft... replacing both "U-joints" would save money and time in the long run.
Be safe... work safe!http://www.rangerforum.com/threads/u-joint-replacement.6756/
UN-Edited response via comment line:
Oh yes indeed.
It appears that the Ranger can come in many different configurations... but the rules of home trouble shooting are to go after the cheap and obvious probabilities FIRST, (loose lug nuts) then on to the rear-end... CV-joints (if you have them)... wheel bearings... finally back up into the transmission.
Seriously... look that drive-shaft over:
Have an assistant hold one wheel while you move your rear wheel... while you are watching the drive-line... the offending u-joint should reveal itself by showing some free-play.
These should be very tight...
Perhaps then when you look more closely you will see the HORRIBLE (chocolate) brown color of oxidized iron... ALWAYS an imminent sign of DANGER anywhere on your vehicle.
The u-joints can be pretty bad... and the vehicle still run... but once that drive-shaft drops your parts costs will escalate vigorously (assuming everybody survives (my drive-shaft didn't... but mine did NOT warn me (silent failure)).
Please do have a look at the URL I posted with my first reply.