2001 4 WD Trans

Hello gentleman , I have a 2001 4 wheel drive QC Diesel, and when backing out my drive way trans works fine , but when I go for Drive it sits there
and does nothing for about 5 sec. , then it starts to move , and works fine the rest of the day , this is not my only vehicle , so the truck might sit for a couple days , I have been told the pump is loosing it's prime , could this be the only answer or are there other answers? , and is it repairable? , If the trans has to come out to be repaired I will go back with a more durable unit , anybody have any suggestions as who makes a good trans and converter? , I don't tow , but I like to play off road , and it has some power mods, (no injectors or turbo) , forgot to say truck has only 81K on the clock and I bought it new , any help would be appreciated.
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I'm thinking that the t/c is draining over a couple of day's.
For a replacement trans and t/c: http://www.dieseltrans.com/ I've used there product and was more more than happy.
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wrote:

You have checked the fluid level right? If not that is the first step. When was the last fluid change? If not recent a change may be in order with a new filter of course. I don't recommend those 'flush' jobs however, but suggest a traditional drop the pan and replace.
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