2001 Mercury Mountaineer Knocking Noise

I had taken my truck on a 250 mile trip pulling my 2 jet skis which weigh around 2,000 lbs. I was about 20 miles from my destination when I felt some knocking coming from the engine. When I arrived and the put the truck in park the knocking was very loud. I immediately turned it off and had it towed to a local mechanic. They measured the oil presuure and it was at 14 psi and should be a at lease 40 psi. They dropped the oil pan and found alot of metal in the pan. They said the crankshaft bearings were gone and needed to be replaced. They said the crankshaft looked good so the bearing and a new oil pump should fix my problem. They installed all the bearings but had a problem with one that was up higher. They damged the new bearing trying to put it on so they had to order a new one. After 4 days and $1300.00 they said it was perfect and ran all there tests and had no problems. I got home with no problems the truck runs great. But now there is a slight knocking when I first accelrate or when I'm driving a slow speed. Once I'm on the freeway I can't feel anything or hear anything. I can feel the knocking in the pedal when it occurs. I put in a oil treament that suppose to seal the rings and it seems to help a little. I'm wondering if maybe they didnt replace that one bearing and that whats causing the problem or if its something else. What do you think?