When put it on reverse it jerks l have this problem sence 2weeks ago my mechanic advise me to get transmission fluid changed but it still jerks when l put it in reverse.

Help! I have a 2009 Ford F-150 XLT V8 6-speed Auto. I pull an 8x10 trailer with lawn equipment on it about 4-5 days a week. I park my trailer at my brothers house when I’m done. It has a slight incline. When I go to back up into the driveway off the street, the engine seems to want to die. Most of the time, I’ll quickly put it in neutral and rev the engine to “clear it out”. Yesterday, I let it go to see what it would do. It cleared itself out without me putting it in neutral or Revving the engine. It seems to happen mostly after a long drive when it’s warm out. We backed up several times to see if we could see any obvious causes. My brother said the engine shook and the fan belt got real slow. He could see the white lettering on the belt (he could see it. He couldn’t read it lol). Spark plugs and coils were changed 2 weeks ago. Any suggestions? Thanks in advance.