2000 Ford Ranger 4x4 XL Cab Feels like someone hitting me in the rear end w/a clunk or like it is the transmission slipping. At times it wants to stall out and the RPMs drop and I have to keep my foot on the gas real light to keep from stalling. Then at times I hit the gas and it revs with no wheel rotation/drive?

ONLY use the identical item as the original equipment.( Don't waste your time looking at other products). If you use a different unit you will have significant issues.

could be brakes... or bearings... check both and repost with more details if that doesnt fix.

pull the pumpkin cover on face of rear-end check for loose or broken parts

Wheel bearings. Or cv shaft bearings. Most likely the cv.

There are no V8s that will bolt right up to your existing drivetrain. The swap would require modification of motor mounts, heater fan box or firewall depending on your motor of choice, a larger capacity radiator, splicing into your existing wiring, a v8 transmission (ie C4 if automatic) and a v8 transfer case if 4x4, and a minimum of a 8.8" ford rear end (preferably a 9" ford rear end)/dana 44 front end to handle the power.

There are many books out there specifically detailing how to swap a v8 into a Ford Ranger. Refer to this site if you are really interested in undertaking this project: http://www.jamesduff.com/broncoII/v8conversion.html

You need to know what the fronts are. Were the rear gears changed?

clunking or grinding noises, wont accelerate to full potential,

The noise is caused by the axle button being worn out, this lets the axle move back and forth and clunk, this is a unique problem with C clip retained axles, it is a noise issue but will not fail and cause any more damage, in order to fix it you must have the axles pulled and the diff carrier checked for excess wear where the axle retainning C clip rides, if the carrier is ok then replace the axles.

Check hydraulic fluid level to be between pointer's marks with engine runing at operating temperature.
See also how to service auto transmition at link below:
http://2carpros.com/how_to/transmission_service.htm