Intermittent 5 speed auto tranny problem

[gustang818]

I have a 2005 Ranger with the 2.3L engine and 5 speed automatic transmission and about 16,000 miles on the odometer. This is my third Ranger but first with an automatic. On occassion when I slow from a higher speed to between 20-30 and then accelerate again (like when coming up to a red stoplight that changes green), the transmission acts like it is in two gears at once. It then "thumps" when whatever it is releases and the truck accelerates normally. Anyone have an idea what causes this and what effect it may have on the tranny's lifespan? I thought it might be something to do with the lock-up torque converter or the tranny actually being in two gears at once. I will post this in the transmission forum also. Thanks in advance for your help.