Yeah, check the trans fluid level FIRST...under the truck. If that's OK, my next guess is the transmission circuit board located on the driver's side of the transmission (under) the vehicle. Your mention of "CLUNKY" shifts is typical of that board having a problem. If it can't determine what gear you're in, the transmission raises pressure, shifts HARD, and (possibly) not when you want it to.
With the motor off, move the shifter fore/aft very quickly several times (to clean the contacts). Disconnect the battery for awhile (or overnight), then reconnect. See if that creates improvement.
2001 Ebony VX and 1989 Custom 383 Corvette
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