what to do now my truck has shifting issues revs high RPMs but will not shift

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ocaasionally it will shift on it own but most of the time I had to put it into neutral then back into drive..i replaced the speed sensor and the abs sensor on the rear axle still no change , then i changed the transmission filter and the fluid, after this the truck abruptly went into gear but still would not shift from first to second , the transmission level would not read (i put in the required 4 quarts) so i added one more now it revs high and will not shift at all not even if I put it into neutral , however it does fine in reverse ..im hoping i do not have to replace the transmission ..any ideas anyone..