The fluid drive is a fluid coupler between the crankshaft and the clutch. It is sort of like a self contained torque convertor, but not exactly because there's no stator for torque reduction as there is in a torque convertor. If it's working properly you can put the trans in gear and let the clutch out while holding the brakes (assuming they work) and the truck won't move. Then if you release the brakes and feed some throttle, off you go.
Your problem may be a fluid level problem. I would recommend changing the fluid. Go to a local farm supply store and get 2 gallons of Universal Tractor Hydraulic fluid ISO 32 grade. Now remove the cover at the bottom of the bell housing and turn the engine over until the plug on the fluid drive is at the bottom. Pull the plug and drain the fluid. It will probably drain slow, so allow it to drain for an hour or better. Now remove the floor boards to gain access to the top of the bell housing. At around the 1 O'clock position you'll find a round cap that may be labeled "Oil Fill". Remove that cap and crank your engine over until you can see the drain hole through the hole in the bell housing, it is now the fill hole. Get youself a long nose funnel that will fit into the fill hole and begin adding oil slowly. Again it will take time to refill it. You'll add oil until it appears full, then wait for it to go down and repeat. Eventually it won't take anymore oil. It should take between 1 1/2 and 2 gallons. Now you have to get the plug back in without dropping it. A magnetic socket or a piece of paper around the head of the plug to make it fit tightly in the socket will help.
If your fluid drive coupling is low on oil, it may have a bad seal. But once you get the lower bell housing cover off, you'll be able to see the clutch and such. Look for evidence of oil leaking between the F/D housing and the "flywheel" where the clutch bolts up to.
And you asked about the shift pattern... Which transmission do you have? Column shift 3 speed, floor shift 3 speed, or floor shift 4 speed?
Merle