I wouldn't waste too much money doing things that "might work" I would just check the fluid level and if it is full then look at finding a low milage transmission or have it rebuilt. At 106,000 miles I would find another one instead of rebuilding, especially if its the cheaper route. And if the guy drained and filled it, most likely it needs to be replaced. $1500 is a pretty good price for a transmission with only 6,000 miles on it. Hell thats how much the transmission for my old Bronco II was.
2001 Ironman Daily Driver... 3.5" suspension lift (OME912 springs and 1" spring spacer), ball joint flip, 1.5" front diff. drop, 33"x12.5 TrXus MT, 16x10 Eagle Alloy rims, Interceptor, PV muffler, K&N air filter, Alpine Supercharger, Bilstein shocks, and some trimming.
2000 Ironman Project LS-1 VX... very slow progress but someday....... ohhhhh someday......